WOW something I can actually help with!! I did that for my Beast wig since the wig needs to be curly, but her hairline is showing in the front.
Basically I wasn't going to get tight enough curls doing it myself so I had to make a lace front for hers and sew it on the front.
But you are able to do it. I got lucky since I had to make it look like the straight hair continued on to the curls, but I could hide the back under her headband. I had to play with it a bit and... I made the lace front myself so it took forever. But it turned out well!
The before is my icon though it's a bit small, and this is the after
If you need like actual help with it I can do that too :3