As you say there is no balances table. Balances are derived from the transactions table.
If you were to insert a row to the wp_wallets_txs table with category=deposit and status=done this would affect the balance. You would need to fill most of the other columns as well, most importantly give your transaction a unique ID. The columns are documented in the schema:
https://github.com/dashed-slug/wallets/blob/3.9.0/includes/wallets-core.php#L293-L313
As you can understand, this is not a functionality that I want to add to the plugin as it would create a mess, but you are free to do this manually if you like.
with regards