There some USB to mini USB adapters. Newegg has several and I'm sure others do to.
You could buy one of these and try a standard USB keyboard.
Mini USB connectors started showing up in cameras several years ago, but I don't think the mini's have ever been standardized. I remember one PC Magazine columnist complaining that even a major camera manufacturer didn't use the same mini connector on all of it's cameras.
Even if it doesn't work at least the adapters are under $10.
Are you sure that the mini USB port can be used for a keyboard, and it just isn't a link to the radio for an MP-3 player.