You can make a macro like this, i use this on my holy priest: where these two are cast ablities on the same talent tree level so only one can actually exist at a time, does not work if both can exisit (not priority based):
do /macro to open your macro interface, make a new one by clicking new, entering a name and selecting an icon then in the macrocommands window put:
/cast Divine Star
That's a /cast followed by the spell name, having both in a row will cause it to cast whichever you actually have.
Make sure you hit save.
Then go to healbot settings -> Spells -> Open the spell selector (the ? next to the input field) and under the macros section select the macro you just made.
Ez pz :-)