Hi Tomemery, welcome to the forum. The rules are simple, be nice and join in!
A .17 hmr CANNOT shoot .22 LR! The Bore sizes and chambers are different. They are entirely different rounds.
http://en.wikipedia.org/wiki/.22_LR
http://en.wikipedia.org/wiki/.17_HMR
I don't know if you could get a .22 LR to fire in your Marlin, but if you could YOU WOULDN'T WANT TO! It would be dangerous.
The .17 hrm is based off the .22 Magnum, so you should be able to just re-barrel to .22 Mag but that wouldn't be worth the trouble (plus the .22 Mag would be only a little more available than .17 hmr). If you have a .22 LR already, just shoot that with the proper ammo and when .17 HMR becomes available, STOCK UP!
I would like a .17 hmr, what do you think of it?
See you around the forum.
Matt M