Its a Doff mission which has a 100% success rate. When it finishes a few random DoFFs are entered into your roster. For one of the missions you can also get a pack of DoFFs which appears in your inventory. That one you can just right click on it and select use to add them to your roster.
Availability wise they are usually found in your home systems and academy areas. For starfleet this is ESD and the Aacdamy, and for KDF it is Qo'nos and their academy.