For a stratovolcano (think St Helens) or something less active, maybe you could pump water down and have it come out as steam the same way a geothermal power plant does. The scale would be huge.
And in either case, the steam would quickly condense into clouds so you would only get a local effect if you wanted to create rain. And to create rain you'd still need a water source which if you had could be used to irrigate. So you'd have to pump some kind of water to wherever you wanted it to begin with. Maybe you can get away with seawater for it, but still a long way to pump it where you want to turn it into steam.
if an important effect of global warming is rainfall reduction and drought...(regardless of costs)
Just use desalination. A very effective yet expensive way to get fresh water.