We often buy dozens of freshly steamed char siu bao from a great source (being out of the way in Chinatown) and freeze them. When we want them, we just pull them out from the freezer, let them thaw overnight in the fridge, and steam. No problem. (If you keep the char siu bao in the fridge a few days instead of freezing them, the meat develops an off flavor.)
I would suggest you steam them initially before freezing the batch.
'Never eat more than you can lift.' - Miss Piggy