Using a 5KVA step-down toroid transformer as a step-up transformer
Assumed facts : Single phase low voltage electrical utility. 

Needs : Has single phase input, 3 phase AC motor controller and 3 phase motor on hand. Can transformer be used to step up voltage.

If you have a single phase transformer (5 KVA) which is rated as step up (or step down) rated for 120 VAC to 240 VAC (or vica versa), can you use said transformer?

Yes, and not much concern for inrush. Part of the fun and learning is just going ahead and trying it. Just be careful. The KVA size could be a problem with inrush current in a home environment (circuit breakers). Use prudence and stand back. You should be ok. Doesn't matter if it is toroidal or not in your application.