I don't think the OP meant he filled up a bank with useless channels. I think he meant that because he only has 8 banks he has to put more than he wants to in each bank and then toggle on/off the ones he currently wants to listen to.
Personally I don't have a scanner that use banks/channels. My scanners use scanlists, and yes I do wish my pro-106 or pro-197 had more than 20 scanlists. So in answer to the OP's questions, I don't know of a way to do what you're asking.
Fortunately for me I don't really have that much of need for more banks (or scanlists) since my needs are quite limited I can get by just fine with what I have.
(Okay, it looks like I took too long to respond and someone else has already answered. Oh, well I guess I just need to learn to type faster).
PSR-800, Pro-197, Pro-106, Pro-30