Okay, I looked all over the place to find a detailed instruction on how to do this safely, with no luck. I ended up incinerating one of my dwarfy kids and melting a tank cart somehow. Here are my suggestions for anyone who would like to do it.
Version 3e: Run 'feature list' in DFHack then type 'feature show #' where # is the entry for 'the subterranean water'. This will allow you to build magma powered buildings.
Version 3f: Just research the liquid spawner as Meph says below, that should unlock your magma powered buildings.
[EDIT] Silentdeth recommended using single tile holes below, which will avoid the magma sloshing out of the pit. Otherwise the instructions will still work, and may even be easier to accomplish. Just plan out where you want your workshops first.
Dig a 3x3 magma cistern, with your magma channels branching off of it, one floor below your workshops (as usual). You'll need magma safe bridges or floodgates to allow the magma in the cistern to fill up first, to minimize evaporation. Or don't, and it will just take longer, depending on your channel size. It may never fill if it is too big. Channel out a 3x3 hole on the workshop floor, over the cistern.
Build (b) a construction (C) called a Track (S)top, setting it to (d)ump toward the hole. Do this several times for faster filling.
Build an Alchemist (other magma-powered workshops can fill tanks too) nearby. You'll want the dorfs to move quickly back and forth.
Forge several Tanker Carts - it requires Metalcrafting at a forge. I recommend Magma Safe, though it shouldn't matter. (Silent explained why magma spilled out of my cistern, and why I melted my cart. Simple fix, but using magma safe materials will still help prevent accidents if you have the materials.)
Set up a hauling route for each Track Stop. Press (h), make a new (r)oute, make a new (s)top. Press (enter) and (x) to remove all the commands in each stop you make, you don't need them.
Select each stop in turn and press (v) to select which cart is assigned to that stop.
Now, if you tell your alchemist to start filling tankers, as they fill, your haulers will haul them to the track stops and dump them. This prevents issues with minecart-related injury/death. If they're only three steps away, it's not a big problem.
Fill your cistern and open the floodgates. Most (all?) magma powered buildings need 4/7 magma below them to operate.
Any tips I missed would be appreciated. It was a big hassle just to fill enough to work with. Especially because I hadn't discovered the use of hauling routes or minecarts yet. I say, do it once to show you can, then use DFHack liquids after that. -.- Maybe this isn't the fastest or best way to do it, but maybe it will help someone.