That modification is certainly doable. Steady hand, pointy needle, strong thread and a general idea of what you're doing is all you need for it. Oh, and a Teddy Babe that's either no longer new (aka softened up) or you'll have to remove much stuffing to be able to do this, especially the sewing up again.
Pick the seams around the vag, then a bit further back to where you want it to sit, and when you start sewing, then do so by starting a bit back in the unpicked seam, so it doesn't unravel on its own. 1cm or .4" served well here for that purpose.
To sew her cavity in again, I'd start at the back end of her crack, use the ladder stitch ofc., move all the way to the front and then around to the back again. Or maybe even stop at the front and start the other side at the back again. Because in such tight places the plush may not stay completely straight and even, and then one side would end up longer (aka more stretched) than the other. Lastly, sew the last bit of the seam at the front closed - and done.
I did a different thing for Lian though, because her inability to spread was annoying me too. I found some sturdy plush, about the same as the new stuff that was introduced by 8W not too long ago (softer than the old material); the color doesn't match, but hey, it's her crotch. Not exactly the high-vis area
Anyway, I cut an oval out of it, with the long ends being rather pointy than round, so that it's a smoother flow of the seams. Then I cut that oval piece into some V shape, and picked a bit of her crack seam and maybe 3/4 way around her vag towards the front. then I started to sew that v piece in where I just had created an open V in her crotch. Now the most crucial (and limiting) point, where those 4 seams connect in her crotch, has much extra fabric, Lian has a real buttcrack now, and can pretty much do the splits now. Downside though - any panty, slip, knickers, strings or whatever she wears slides up - good she's not human to complain about it. Just think of "butt floss"
Now for the sex positions: for rear entry I leave the inserts I use sticking out a bit, then pull it back and between her legs, which I tell her to squeeze together. Voilá, works like crazy, for spooning, doggy etc etc.
We prefer Teddy Babe on top though, most of the time, and for that she's been perfect from the start. You know, I am a lazy bastard, and that position is so comfy...
She owns you, after all. As does Amy. Try her against a wall for fun and watch yourself completely lose control faster than you'd like... 
