I had considered this too and have an old espresso maker, but the
pressure is lacking compared to a real steam cleaner. It will work
to use the milk frothing attachment, just not great.
Another thing I came across, but haven't tried yet: I had bought a
bunch of old (and I mean OLD) tools off of a jeweler in my area for
a jewelry class I was assist. teaching. In one of the boxes was a
hot plate steam cleaner. I had to call him back to figure out what
it was. I'll try to explain how it was built and how it worked,
but I never used it, so I don't know how well it worked.
It was made with copper water tubing, like what you hook up to a ice
maker. About 1/8 inch. That hooked directly into a faucet or water
line. The copper then ran into a metal housing ( think large
upside-down can) and spiraled about 20-30 times around and came out
the top of the can. The copper then bent down and the end was
smashed and soldered shut. A small hole was drilled into the end of
the copper tubing. The housing sat on a hot plate (which stayed on)
and when you needed steam, you just turn on the water.
Like I said, I've never tried it, but would only cost about $10 to
build. If anyone has used one of these, I'd love to know how well
they work .