E-mail woes

So, I have four e-mail accounts synchronized with Apple Mail, with IMAP, and the iPhone with IMAP, and I want to consolidate them all into one Gmail account, so that I'll only have to deal with one inbox on the iPhone. That would all be fine and easy, but here's the catch. I want Gmail to remember the e-mail accounts from where the messages were sent to, and respond with the same account, from Mail or the iPhone, not just the web app, which does this perfectly. Is this even possible to do with IMAP? As far as I know, it isn't. I also like how Mail can have different signatures for different accounts, and that's an advantage to using the program over Gmail's web app; Gmail doesn't have multiple signatures for different accounts, even in their web app, which is pretty weak. I don't know if the IMAP protocol is this sophisticated. I doubt that there is such a mechanism at this time.

Until a solution pops up, I'll just complain. I always seem to have the most esoteric technical problems.