I don't think the n900 has a USB port. I think it has a micro-usb port so you would need a converter that has a male (something you can put into the n900) end that's micro-usb and the female (something that lets things be plugged into it) end that's USB. Then plug the USB keyboard into that.

For example, for the nx00's we've had the OTG adapter being sold on here for a while that does that. It's useful for when I want to plug my USB drive into my n800.