I don't begrudge a Star Destroyer for being a mere kit-bashing of pre-existing models.
Re-use code, art, text, everything. The purpose of the challenge is the creation of interesting new roguelikes. The easiest route to this is often the vivisection of an existing roguelike :> Also consider things like libtcod which make A* a mere library call.
As for prior work, the goal is to not write any code; or any text; or any art; *for* your specific planned game prior to your chosen start date. This is an enforced brain-storming time to turn over ideas in your head or paper and plan stuff out. So feel free to write down your ideas or paint concept art, just not as code or final sprites :> Do be careful your scope does not get too large... I'd also recommend doing some adminstrative checks before the start. Make sure your compiler exists, version control is working, etc.
The purpose of the 7 day window isn't actually to prove some macho ability to code quickly. 7 days is too long for that, IMHO. It is more like the NaNoWriMo month time limit - it is short in order to drive you to actually complete a game. Prior to the 7DRL challenge, way too many roguelikes would flounder in a half-baked dream state; wishing to some day be a NetHack but never actually getting to the "game" part. With a 7 day window you have to just do it; and must not procrastinate or let dreams pile on dreams.
So have fun and good luck! And feel free to copy & paste your A* code :>