This is the Hydrasynth so sure you can do this. There is no “drone mode” or “midi hold” that i know of.
But here’s one way to do this, though i don’t know if it’s the best. There are no doubt other ways.
I started with the Init patch. Then i set Envelope 2 Hold (Env 2, Page 1, #6) to max of 36 seconds, Freerun (Env 2, Page 2, #7) to ON, and Env Loop (Env 2, Page 2, #8) to Infinity. There is a very small click at the end of the hold time, but you won’t hear it unless your wave form is something like a sine wave.
No doubt there are other ways to do this.
You can be running modulations with this, so for instance do this and set up a nice talking Wavescan that you walk semi-randomly through.
I also started the arpegiator on top of this. Or tweaked the ribbon controller. Or played keys and replaced voices making the drone simpler or more complex.
ADDED: For fun take any random patch and do the three things i list above to Env#2. Then play with the Macros or the filter or whatever. This synth is always blowing my mind.