I've done this with a normal camera. It is possible if you use an external device like this one which I used. It just copies files and works very well. I've copied >1 GB from a normal camera to N900's external microSD card without problems.

This thingy is very cheap so you can try it without loosing anything.

Of course, it should also copy anything between two usb devices (e.g. N900 and usb stick or camera and usb stick).

NOTE: This is not related to what other threads say about making N900 usb-host. The usb-host in this case is that simple device.