Is there a way to make your level 20 perma pre as a merc?

I wish they have a Merc Registrar in pre-searing but they dont.

afaik you are not able to use heroes (merc or other ones) in pre searing, so making a merc registar in pre searing has no use...

you also cant use yourself as merc

EDIT: i forgot you have to show yourself to a registar before other chars can use you as a merc hero. so yes, that is a shame that you cant do that with pre-searing chars

sorry. in pre there is no hero option so nope. you can't do that.

Although it would be nice, but in a way also a bit contradicting ..
Your character only exist before the searing, he couldn't then be a mercenary hero in post searing.

Thank you guys, I guess as much.

It is ok if pre doesn't have a hero option, but you can still have a merc registrar without having a hero option. A merc registrar registers you as a merc for your other characters to use you as a hero, not to give you the hero option itself.

Alternatively make a PvP character that looks like your pre char and make name as close as possible.

A more accurate answer would be that there is no Mercenary Registrar in pre-searing - which, apparently, the OP already knows.
So, I think you would have to leave pre and travel to a Registrar's location, in order to register that character as a merc - which kinda defeats the perma-pre aspect. Until/unless they add a Registrar to pre, the above idea of making a PVP character is probably the best option.