Yes, you can join the French Foreign Legion, U.S military, U.K
military & a few others. But I know for certain the Israeli
military won't take in non-jews.
The thing is, except for the French Foreign Legion, you've to be a
citizern of the country inorder to join the military proper.
Otherwise, you'll be assigned to non-combatives duties. This means
that you can't have 2 citizernship.
Hmm! I don't think you can join a foreign military just for the
purpose of experience & then returned to your country of origin
to serve back in its military. The foreign military can prosecute
you as a spy, while your own country military can't trust you.
Also, theres the age factor involve. When you join a foreign
military, example, U.S Marines, you'll have to go through the basic
trainning even though you've done it in your own country military.
After basic trainning, you'll have to see which unit you'll be
assigned to. Then you've to go through advance trainning in the
unit you are assigned to before you can be certified as combat
ready. So by the time you've finished your tour of duties with the
Marines, I don't think you'll want to wear another unifor again. It
can be quite depressing. 