Hi Helms - this is a good question, I don't have the proper answer but I've come up with a solution that seems to work for me:
I add all of the grain-based fermentables into my Beersmith recipe (malt, grains & related), choose the appropriate yeast (Beersmith calculates Attenuation based on the chosen yeast)... then note the Estimated FG, write this down. Now add the sugar to your recipe - note how the Estimated FG increased...? What's up with that btw? Anyhow, I just use the Estimated FG from the pre-sugar recipe and add a point or two to the FG based on the type of sugar.
Calculated Final Gravity Estimate (from grain/malt syrup additions) = 1.009
Calculated Final Gravity Estimate (after adding 1lb Sugar) = 1.012
Table Sugar adds very little in the way of complex 'unfermentables'. So I just adjust my FG estimate from the pre-sugared recipe... maybe 1.0095 in this example. Now if you're using a sugar with a higher percentage of unfermantables (maybe Molasses or Demerara) - these will contribute a little more to your FG... so maybe 1.010 in this particular case.
Someone please tell me there is an easier way to do this! I see that certain ingredients in Beersmith have something called "Dry Yield" but this only seems to effect the OG contribution. *Doh* WTF?