I did something like this on a couple guitars. I bought a set of 7 string guitar strings and used the B thru B strings on a 6 string guitar. I needed to do a little widening of the nut slots but other than that it worked fine. I figured if I was using 10's on a six string guitar, I'd buy a 7 string set of 10's and the tension would feel about the same to me and it did. From the D'Addario site for XL's.
[TABLE="width: 384"]
[TR]
[TD="class: xl64, width: 64"]Item #[/TD]
[TD="class: xl64, width: 64"]Note[/TD]
[TD="class: xl64, width: 64"]Inches[/TD]
[TD="class: xl64, width: 64"]mm[/TD]
[TD="class: xl64, width: 64"]lbs[/TD]
[TD="class: xl64, width: 64"]kg[/TD]
[/TR]
[TR]
[TD="class: xl63, width: 64"]PL009[/TD]
[TD="class: xl63, width: 64"]E[/TD]
[TD="class: xl63, width: 64"]0.009[/TD]
[TD="class: xl63, width: 64"]0.228[/TD]
[TD="class: xl63, width: 64"]13.14[/TD]
[TD="class: xl63, width: 64"]5.96[/TD]
[/TR]
[TR]
[TD="class: xl64, width: 64"]PL011[/TD]
[TD="class: xl64, width: 64"]B[/TD]
[TD="class: xl64, width: 64"]0.011[/TD]
[TD="class: xl64, width: 64"]0.279[/TD]
[TD="class: xl64, width: 64"]11.02[/TD]
[TD="class: xl64, width: 64"]5[/TD]
[/TR]
[TR]
[TD="class: xl63, width: 64"]PL016[/TD]
[TD="class: xl63, width: 64"]G[/TD]
[TD="class: xl63, width: 64"]0.016[/TD]
[TD="class: xl63, width: 64"]0.406[/TD]
[TD="class: xl63, width: 64"]14.68[/TD]
[TD="class: xl63, width: 64"]6.66[/TD]
[/TR]
[TR]
[TD="class: xl64, width: 64"]NW024[/TD]
[TD="class: xl64, width: 64"]D[/TD]
[TD="class: xl64, width: 64"]0.024[/TD]
[TD="class: xl64, width: 64"]0.609[/TD]
[TD="class: xl64, width: 64"]15.73[/TD]
[TD="class: xl64, width: 64"]7.13[/TD]
[/TR]
[TR]
[TD="class: xl63, width: 64"]NW032[/TD]
[TD="class: xl63, width: 64"]A[/TD]
[TD="class: xl63, width: 64"]0.032[/TD]
[TD="class: xl63, width: 64"]0.812[/TD]
[TD="class: xl63, width: 64"]15.5[/TD]
[TD="class: xl63, width: 64"]7.03[/TD]
[/TR]
[TR]
[TD="class: xl64, width: 64"]NW042[/TD]
[TD="class: xl64, width: 64"]E[/TD]
[TD="class: xl64, width: 64"]0.042[/TD]
[TD="class: xl64, width: 64"]1.066[/TD]
[TD="class: xl64, width: 64"]14.37[/TD]
[TD="class: xl64, width: 64"]6.52[/TD]
[/TR]
[TR]
[TD="class: xl63, width: 64"]NW054[/TD]
[TD="class: xl63, width: 64"]B[/TD]
[TD="class: xl63, width: 64"]0.054[/TD]
[TD="class: xl63, width: 64"]1.371[/TD]
[TD="class: xl63, width: 64"]13.25[/TD]
[TD="class: xl63, width: 64"]6.01[/TD]
[/TR]
[TR]
[TD][/TD]
[TD][/TD]
[TD="class: xl65, colspan: 2"]Upper 6 (E to E)[/TD]
[TD="class: xl65"]84.44[/TD]
[TD="class: xl65"]38.3[/TD]
[/TR]
[TR]
[TD][/TD]
[TD][/TD]
[TD="class: xl65, colspan: 2"]Lower 6 (B to B)[/TD]
[TD="class: xl65"]84.55[/TD]
[TD="class: xl65"]38.35[/TD]
[/TR]
[TR]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD="class: xl64, width: 64"]Item #[/TD]
[TD="class: xl64, width: 64"]Note[/TD]
[TD="class: xl64, width: 64"]Inches[/TD]
[TD="class: xl64, width: 64"]mm[/TD]
[TD="class: xl64, width: 64"]lbs[/TD]
[TD="class: xl64, width: 64"]kg[/TD]
[/TR]
[TR]
[TD="class: xl63, width: 64"]PL010[/TD]
[TD="class: xl63, width: 64"]E[/TD]
[TD="class: xl63, width: 64"]0.01[/TD]
[TD="class: xl63, width: 64"]0.25[/TD]
[TD="class: xl63, width: 64"]16.22[/TD]
[TD="class: xl63, width: 64"]7.36[/TD]
[/TR]
[TR]
[TD="class: xl64, width: 64"]PL013[/TD]
[TD="class: xl64, width: 64"]B[/TD]
[TD="class: xl64, width: 64"]0.013[/TD]
[TD="class: xl64, width: 64"]0.33[/TD]
[TD="class: xl64, width: 64"]15.39[/TD]
[TD="class: xl64, width: 64"]6.98[/TD]
[/TR]
[TR]
[TD="class: xl63, width: 64"]PL017[/TD]
[TD="class: xl63, width: 64"]G[/TD]
[TD="class: xl63, width: 64"]0.017[/TD]
[TD="class: xl63, width: 64"]0.43[/TD]
[TD="class: xl63, width: 64"]16.58[/TD]
[TD="class: xl63, width: 64"]7.52[/TD]
[/TR]
[TR]
[TD="class: xl64, width: 64"]NW026[/TD]
[TD="class: xl64, width: 64"]D[/TD]
[TD="class: xl64, width: 64"]0.026[/TD]
[TD="class: xl64, width: 64"]0.6604[/TD]
[TD="class: xl64, width: 64"]18.38[/TD]
[TD="class: xl64, width: 64"]8.34[/TD]
[/TR]
[TR]
[TD="class: xl63, width: 64"]NW036[/TD]
[TD="class: xl63, width: 64"]A[/TD]
[TD="class: xl63, width: 64"]0.036[/TD]
[TD="class: xl63, width: 64"]0.9144[/TD]
[TD="class: xl63, width: 64"]19.04[/TD]
[TD="class: xl63, width: 64"]8.64[/TD]
[/TR]
[TR]
[TD="class: xl64, width: 64"]NW046[/TD]
[TD="class: xl64, width: 64"]E[/TD]
[TD="class: xl64, width: 64"]0.046[/TD]
[TD="class: xl64, width: 64"]1.1684[/TD]
[TD="class: xl64, width: 64"]16.91[/TD]
[TD="class: xl64, width: 64"]7.67[/TD]
[/TR]
[TR]
[TD="class: xl63, width: 64"]NW059[/TD]
[TD="class: xl63, width: 64"]B[/TD]
[TD="class: xl63, width: 64"]0.059[/TD]
[TD="class: xl63, width: 64"]1.4986[/TD]
[TD="class: xl63, width: 64"]15.87[/TD]
[TD="class: xl63, width: 64"]7.2[/TD]
[/TR]
[TR]
[TD][/TD]
[TD][/TD]
[TD="class: xl65, colspan: 2"]Upper 6 (E to E)[/TD]
[TD="class: xl65"]102.52[/TD]
[TD="class: xl65"]46.51[/TD]
[/TR]
[TR]
[TD][/TD]
[TD][/TD]
[TD="class: xl65, colspan: 2"]Lower 6 (B to B)[/TD]
[TD="class: xl65"]102.17[/TD]
[TD="class: xl65"]46.35[/TD]
[/TR]
[/TABLE]
