If you have to ask how to do it then you probably aren't qualified to do it. To be honest I doubt anyone here is, though a few may be smart enough to work it out. The cellular modem firmware is included in the FIASCO firmware image, if you know what you are doing you'll be able to extract it, decompile and reverse engineer it to the point where you can hack a modified version that does what you want.

But really, if there is one thing that I would trust Nokia to do well, it is making the cellular modem work properly. From my experience my N900s work better than my sisters' iPhones in poor reception on the same networks so I don't really have any complaints. I don't find it to be draining too much power either (although I do live in a good reception area).