To my knowledge, the Support system is very hardcoded. I don’t think you can edit Support ranks themselves.
However, there is a patch called something like “Optional talk with A support”. Which lets you trigger events with a condition being 2 characters having an A support together.
With this, you could recreate the S support system, if what you want is child units, by making the Parent who’s tied to the child (Mothers in Awakening, Fathers in Fates), have multiple optional A support talks, and make them all have the same completion flag, so you can only trigger one of them. And you may need a few extra flags to keep track of who’s married to who.
Additionally, if you can handle some ASM (I can’t), you may wanna look into Snake’s Support Rework Rework, which supports 8 support levels