I doubt OpenMPT is going to give you sample-drawing capability. There are, however, a couple of ways around this:
1. Do your sample-drawing in an external tool (like Milkytracker), and then load these into OpenMPT.
2. Get yourself a VST that supports sample-drawing (a quick search at KVR turned up
this, which I haven't tried, but is free, and might do the job).
3. Use VSTs to generate the sounds, rather than drawing the samples directly.
If you
really want to see this in OpenMPT, you can probably open a feature request at the
Modplug forums, but as I say, I'm not convinced that it'll get implemented.