Hey, sure, that's pretty easy to do using 'aconnect' ALSA utility. After plugging in the USB MIDI controller to the Raspberry Pi,
aconnect -l can be used to list the ALSA midi ports, and then something like:
aconnect 12:0 20:0
aconnect 20:0 12:0
would forward the MIDI data to and from pisound's serial MIDI ports to the USB MIDI device. The 12:0 and 20:0 numbers would be based on the output of
aconnect -l command.