RainmanP

Cat,
Go over to sheldonbrown.com and read his article on this. If I remember correctly, he specifically discusses the issue of using one cassette on the other system. If you are lucky, the hub may be Uniglide. If it is Hyperglide, it will have one wider spline which is used to correctly position the cassette or individual cogs you may replace later. If I remember correctly SB says you can grind one of the notches on the cogs and spacers wider so they will fit on the HG freehub. Don't take my word for it; go read the article. You will learn a lot. I think it would be easier to get an HG cassette. 7-speeds don't cost much - $20-30 for HG70 (105), a little more for Ultegra and DA.

Is your aim to assemble a pure DA group? Because you can purchase a 7-speed freehub body and slap it on just about any Shimano hub. I just ordered one from Harris Cyclery to use on a new 105 hub to use in place of the original 6-speed freewheel rear on my old Bianchi.

I think I would use an HG-70 cassette (steel) for most of my riding and only put on the DA (aluminum) for actual racing. It seems like the aluminum would wear out faster and they are much more expensive to replace. Cassettes are expendables. I don't think using an HG-70 cassette makes it any less a DA group for day to day riding.
Good luck,
Raymond