Unforgiven:
Yes, as Ryan pointed out, it's a regular practice. The only code violation that I can envision would be the length of the conductors may be to short when installing multiple extension rings.
What I'm saying is, if the conductors in the original box were 4" extended past the front of the box, and you put two (2) 1-1/2" extensions on, then it could be a violation.
John
