To add to an Admiral's fleet, park the Admiral in a port and build new ships right into his fleet.

The weird thing is that when you get into open water and break up the fleet, I can never figure which ship is going to end up with the Admiral. It's not necessarily the largest, or even the first in the stack. The Admiral seems to stay with the largest stack and then when you are down to two ships I can never get it right... I always end up with the Admiral on the no chevron Bireme.

By the way, if on of your Admirals develops a command star then loses his only star in an unexpected defeat, the bugger still won't combine...like he's worth anything to your Navy on a stupid bireme with no exp and no stars.