Typically you charge a Pb battery to 2.4V per cell so a 21.6V power supply. Most 24V power supplies are adjustable into that range. All you have to do is limit the current to a point lower than the output current of the power supply.
Say you have a 24V power supply rated at 5 amps You would need a resistor rated at about 5 ohms and 120W. That's a little on the large side and a pretty high charge current. about a 1 to 1.5 amp rate is a little better for up to about 12 Ah. So you would still need 5 ohms but only about 40 watts. Basically the resistor would limit the current coming out of the power supply and keep it from tripping or just smoking.
http://www.mpja.com/prodinfo.asp?number=16008+PS
Jeff