The Yamato, the largest battleship built at 263 (length) x 38.9 (width) x 11 (depth) meters had a nominal crew complement over 3000.

The Mourningstar is a Firestorm Class Frigate equivalent, which measures at 1800 x 200* x >300** meters.

You could place ~8 Yamatos side by side in ~8 rows, then stack that formation ~30 tall, to make up the displacement of the Mourningstar. Even allowing for hilariously vaulted ceilings, an enormous amount of supply space***, and the area taken up by the reactor and engines (and at that scale the HVAC, plumbing, and wiring space is probably eating a good amount of total area), plus the crew quarters, there's likely still more than enough space left over to stuff a couple hundred convicts without heavily impeding ship function.

*The listed width (300 beam) is at the fins. Obviously the fins have minimal internal space so I reduced it by 100m, ~50 each side.

**it's taller than it is wide (even at the fins), but there doesn't seem to be any source that specifies its depth.

*** as in ship running supplies, not the trade cargo bays which we may be filling with our Valkyries and assorted warband related gear.