How Can I do This?
or, do what I did, just relearn whatever songs you are doing on your downtuned 6 string on the 7 string, in the old band, we used to play all kinds of stuff that was Drop D, or downtuned, and thats where I totally fell in love with my 7 string, because with not a lot of work, I could relearn everything and play it all on the 7. *Some of the songs required some really odd fingerings, but I got them down, and it was all kewl. *The thing of it is, if you are playing a 6 string that is just dropped one step, all your barre chords and everything are still the same shape, and if you can learn the new positions on a down tuned axe, then you can surely learn them on the 7. *
As for drop D, some stuff was tricky to pull off on the 7, but it's all do-able, we did a lot of Motley Crue, and it's all drop D pretty much, but I never once had a Drop D guitar.