Oh heavens. That sounds like it would be superfluous and complicate things more than necessary (as you said.)
Suggestions... Off the top of my [tired] head, something like Karif's suggestion seems a far simpler approach-
.. but you're right when you say this-
I suppose it depends on how you want to do it?
If we can't say 'Boop! You're now at level 6!", we need to find a way for people to provide evidence, to prove the character has earned the right to be at level XYZ. Since this forum is at its beginning... there's no hard evidence here for players to use to prove their character shouldn't be starting at square one. So.
Based solely on gameplay... the only way I could think of would be to link rider accomplishments on the sister forum to this one- only as an initial way of gauging where an established characters fits in, a one-time-only deal; it certainly wouldn't be ongoing and couldn't be used as a way to 'stack' points.
For example: Say I want to introduce Johnny Doe into the system, and my rider records say he's entered 60 shows: if entering a show counts for 10 XP points in this system, and we count those 60 shows on the other forum, that could grant him 600 XP points, and he'd have "earned" his place at Level 4 in this new system.
... but I don't know, perhaps you'd rather not bring the other forum into this? I don't want this to turn into a lot of work for some poor admin-type, and I certainly don't mean to make this more complex than it should be. I can see how it could potentially be a headache.
A possibly simpler alternative: linking to past posts which show the capabilities of the character in question? Be it pictures or writing... although again, I don't want to send someone on a verification treasure hunt. Maybe we could think of some sort of capability test, a checklist of criteria, which 'backstory characters' can take in order to find their place?
[poor] Example: in order to be proved a level 3 trainer, Jane Citizen needs to have conducted one public clinic, trained with four private clients, and competed herself with two different horses. If the member can provide evidence- pictures or writing (or other?)- which prove she's met the criteria, then she rightfully earns level 3.
.. man, so many words, so little regulation of thought.
... Simpler still could be saying 'screw it' and having them either start from level one (like everyone else) or take a back seat. It is an opt-in system, after all, and personally, I won't be bothered if these characters with history aren't in the system.