How do I have Alternate Form increase a non leveled Advantage?

[Jareth Valar]

I'm looking to have an Alternate Form increase a non-leveled advantage. I know if you have a leveled advantage like Hard to Kill or something, the levels automatically stack.

I'm looking to have an Altered Form increase the level a character has in Regeneration and Resistance. Right now I just created a New Advantage and wrote in the description with a static Limitation, but I was wondering if there is a "proper" way and one that will increase the Advantage depending on if the character has the ability outside the AF or if different characters have different starting levels.