Can I still add store credit to a customer wallet that is frozen?

Yes. Freezing blocks spending and top-ups, but not credits. While a wallet is frozen:

• the customer cannot pay with it at checkout or top it up
• you cannot debit it
• you can still add a Credit under Manual adjustment in Plugins → Wallet → Customer Wallets → Manage

So you can credit the refund now and the balance waits safely. When the matter is settled, click the unfreeze button in the same window. The status in the list changes from Frozen back to Active.