Choice of abilities within an ability list

[cryptichorror]

I've figured out how to make a list of custom abilities within a class I am designing. I need to go one step deeper, however. One of the custom abilities that the player may choose allows them to choose a combat maneuver to master. How do I add a drop down list within the custom ability to allow them to choose? Also, this custom ability can be chosen more than once, so I need the combat maneuver to only show up if it hasn't already been selected.

 

[cryptichorror]

(if this was in a tutorial, I missed it.)

 

[Mathias]

I'd add 7 or so separate versions, each one specific to a maneuver.