It looks that the game has been somehow already prepared for the fleet officers: So to attach officers might be similar to the diplo case with similarly calculated time for their travel.
I would propose as a first step only to increase the fleet experience every turn, of course based on the leader characteristics.
And in the next step to add some bonuses for the battle, like attack, defense, manoeuvrability or command ones.
But we might discuss the details in a separate topic.

