What do you mean by "dumb" here? That it doesn't make sound on its own?
If so, they're they only kind of keyboard I've ever owned, and if its class compliant Midi over USB, it just pops up in the ALSA tab of QJackCtl.
Using the MIDI driver selection in QJackCtl's setup dialog, you can specify "raw" or "seq", both will show the ALSA soundcards MIDI events in JACK MIDI.

If you're still doubtful, what's the make & model of the keyboard? Perhaps somebody has experience with that exact hardware.
HTH, -Harry