Someone who's actually put together should chime in but from what I remember, the axle on the Poseidon attached to the main hull and engine block via a thick '+' shape. No such shape exists on the Omega front or rear hulls to match up with that shape.
One could bore out a hole and cut off the segments on the axle with the + shape, but then the axle would be too short to insert into either Omega body segment. Alternatively, you could file off the + shape, but don't know how secure that would be since the axle is bearing so much weight.
If one is going to make the center part spin, you'd have to buy a brass rod, bore out the fore and aft body blocks to fit, cut to length and glue...