SnugglySim, with all due respect, if you don't want to take the time to do what you're asking properly, then don't bother asking at all. I can help you to do what you want, but you
are going to have to use SimPE. The instructions are simple and easy to understand (I've even included pictures in my tutorial that I'm going to link you to).
First of all, PirateBooty has posted some fairly straightforward instructions on how to change the genetics of features (hair, eyes, etc.) in the game as a whole (as in, making the eyes/hair/skins genetically dominant/recessive for everyone and not just your one Sim), so if you want to change the dominincy/receccivity of the hairs in the game in general (and not just for the one Sim), go ahead and do that first.
Secondly, I have written up a tutorial for changing the genetics of NPCs which might help you out (it's found
here). However, to simplify things, here's what you can do:
1. Clone the Sim (since you mention this, I assume you know how to do this).
2. Create the Sim in CAS (making certain to give her the custom hair so that it'll be considered dominant when you create her), leave her in the SimBin in the appropriate neighborhood and then exit the game.
3. Start on step 5 in my tutorial that I've linked you to (and
end at Step 14). This will allow you to keep everything the same and will give you the means to make her custom hair the genetic property for her.*
The nice part is that in your case, you won't have to use SimSurgery, since she already looks the way you want her to look.
Good luck.
*In fact, I think you'll only need to copy/paste the one line, as the Hair's the
only genetic property you want to change, yes?