Take some time and sift throught last year's threads on USB-host. This has not changed since the early days, and all the answers are there. The very simplest way I know of (and use) is to find a "Y-shaped" USB cable, the sort you use to bring the juice from two host ports to a single device. The trick is to use the "device" plug in the "host" port of the Nokia.
On the "juice-only" port connect a 4xAA or 4xAAA battery pack with a USB plug ; on the third one connect your keyboard, mp3, or USB hub and you're set.
Oh yes, and USB genders and formats are hell :-)