Ring: of Greater Fay Dark?
 

Is it possible to make a spell,

such as take a spell NOT in use , and
make it A Circle of Greater faydark or Ring of Faydark ??

and make them spawn beside the Ring's in Gfay ?

Yes.
As to how I am not sure but I know you can do it.
If you could look at a "normal" port I am willing to bet you will see XYZ cord in there somewhere with a zone id number.
Find the zone id you care to have your new spell go to and the XZY and test it out.

Ranger's Tolans BP has clicky port to Faydark Ring

you can either use that OR simply make copy of it

If it exists as a spell in game already, wouldn't it be as simple as allowing player classes to use it and creating a scroll to scribe it?

I used , Spell: grimling forest and swapped it

yes, unless he specifically doesn't want to alter any original spells in any way.
for whatever reason =)