Thanks, guys!
BMo: The joints are from a Star Wars Ahsoka Tano figure. Ahsoka's shoulders are going to April's elbows; the knees are the knees.
gunstarheroblue: The cool part about it is that the cuts don't have to be clean or measured out. The way I did it was to make the Ahsoka pieces longer than necessary and make them narrow at the ends, then cut April wherever and drill holes in the April parts. That way I could slide the Ahsoka parts up or down as necessary (comparing them to the original leg at first, and now to the modified leg) to get the desired length/knee placement.
And yeah, I'm going to leave the boots as is, since it doesn't look like anyone else in the line is getting ankle articulation. I kinda want to keep April looking as "factory" as I can -- so basically I'm giving her the articulation Playmates should have (what the Turtles got), but not going beyond that in terms of modifications and painting.
Though I guess, since April's got that waist cut, she'll surpass the boys by one POA. Girl power?