Not directly, but the quick workaround is to do something like build the recipe with 8.0 lbs 2-row, 1.7 lbs crystal, 0.2 lbs chocolate and 0.1 lb wheat (i.e. take each and make it the percent/10) and then use the "adjust gravity" function to adjust the recipe to have the target final gravity. This will preserve the percentages.
Cheers,
Brad