The custom transactions are not included in the lisk-client.
Therefore you still need to import the custom transaction that you want to broadcast.
Based on this you can create the Transaction JSON object.
This object can be send to the desired node by utilizing lisk-client.
One example how to broadcast transaction with the Lisk-APIClient is described here: https://lisk.io/documentation/lisk-sdk/guides/app-development/broadcast.html#post