Okay steps 17-25 in Brianna's mesh creation tutorial sort of apply.
http://www.modthesims2.com/showthread.php?t=43092Where the file you are extracting the resource node and shape from is the mesh file for the mesh you want to bring over.
Of course, you also have to know which 3D ID Reference belongs to the age in question.
Generally I believe the 3D ID reference files with instance 1 is toddler, instance 2 is child and instance 3 is teen. It's starts to get stickier with 4, 5, and 6 because in my experience they don't seem to be consistant between different packages, or maybe I just have some messed up hair packages heh.
Unless the hair uses a Maxis texture and is pointed to use the Maxis texture (the only people I know of who have done this are myself and Pinhead and possibly Morague so I'd say the chances are slim to none) then you'll have to bring the textures over too. I think the Binary Indexes can be left alone but to be honest, I'm not even certain about that, we'd have to ask Pinhead on that.
Basically, I don't mean to spoil your wish, but it's a huge pain in the butt and kind of makes my brain explode just trying to explain it lol. Plus, I haven't had much luck with hairs I've been trying to mess with lately so Pinhead would be a better person to talk to about it, but be warned if you REALLY want to do this, there is no easy automated way, it's all manual editing.
I'm not afraid of having to do it manually, as long as I've got correct instructions. (First mods I made were two custom careers done the long way, before I knew there was a career editor.) XD
I did try replacing the 3D ID Reference, but I either did it wrong or that doesn't work, because it had no effect. :p The mesh in question does happen to use a Maxis texture, though, for a wonder--it's just the adult ponytail scaled down for kids. Heck, for all I know, one of you guys may have made it, I lost track of the creator. XD