Well . . . It would still have to be one of your RP abilities - the custom ones too in this case. Otherwise you wouldn't have been able to do that in a real RP over the forum.
No, you can - except in real RPs . . . unless of course you've set your character to be that way. So I suggest making all your moves to fit your job as an elementalist.
Go to your character sheet and just customise the abilities (the set ones) to fit your character and then make up your own 4 abilities by filling out the table thing.
I'm the Honorary Deputy Leader and I have full control over the members of the group. I could make you one right now or kick you out when you've reached 1000 posts XD