Company has signifigant foreign operations employing ex pats. They would like to add an after tax provision to existing 401k plan ( 401k contributions are not viable as most of the income earned overseas not subject to American taxation up to a certain limit). Would it not be better to just add the Roth 401k? Does anyone have any experience with this type of arrangment?

Why do you think that the 415 definition of compensation would be different for after tax and Roth than it is for pretax? Doesn't the exception for 911 compensation work for you?

I don't think the issue is defintion compensation. My greater concern was allowing ex-pats to contribute and the associated administrative issues. Testing concerns aside, any reason not to accomplish this with the roth -k?

Roth 401(k)s are not effective until January 1 2006. And many won't implement until later than that.

So if your question is purely administrative, you might not get any takers for a while.

"Testing concerns aside" makes your question trivial. If you don't have to comply with the law, anything can be adminstered as a matter of book keeping.