a generic usb midi or audio solution, such as available for windows, does not exist for OS9.
with akai you might have luck, some of the older, "first generation" controllers do still support OS9, i have an mpd-16 for example which comes with a device driver and programmer app. there should also be some products from edirol, novation, which were also available at OS9 times.
to be honest i dont see any advantage of usb-midi, i am happy with doepfer & co here, it is easier to manage MIDI DIN ports in OMS when you need some flexibility - and it does not cause the usual running-out-of-ports problem which you always have with usb and firewire.