First, it's actually a full (and sent) transaction that creates the unspent transaction output containing the right quantity of the asset/s. This output is then used as an input for the exchange transaction.
In any event, you can unlock this transaction output by calling:
lockunspent true '[{"txid": ..., "vout": ...}]'
Replace the ... with the appropriate values returned by preparelockunspent.
(Note that it's surprising the parameter is 'true' above, but that's the right value.)
Note that the lock is only something stored in the memory of the local blockchain node, to prevent an output being spent unintentionally. It has no effect on other nodes in the network and is not a fact recorded into the blockchain.
