Yes, MM-15 will support any MIDI keyboard as a simple keyboard controller for laying down MIDI tracks.  MM-15 does not support pitch-bend/mod-wheel messages or other synth controls; only note ON/OFF, velocity, volume,etc.

The resulting MIDI tracks can be played in MM-15 using any of the internal VST instruments, or any other imported VST instruments.  Any modulation, etc. within the VST instrument itself is supported (not dynamically by the keyboard).

Since I don't play keyboard very well myself (yet), I've not tried it, so I am not an expert and you can take it for what it's worth.

Obviously, you'll need a MIDI and/or USB interface of some kind.  Your USB connection is the simplest solution.

If you are using a synth that has its own voices,  you'll need to set it to LOCAL/OFF so that it behaves as a controller.

In MM-15, you'll need to set the MIDI settings (Audio/MIDI tab) in the Program Settings ('Y' key).

Beyond that, you should read those sections of the manual to make sure I haven't missed anything.  There's an index at the end that makes it easier to find the relevant section(s).