I have a plan sponsor that has an existing 401(k) safe harbor plan using the 3% non-elective contribution. What they want to do is to have separate eligibility provisions for the 401(k) portion of the plan and the 3%. Specifically, they want to allow new employees to become eligible to start making salary deferrals immediately upon employment and become eligible for the 3% non-elective contribution after 6 months of service.

Can this be done? Are there any pitfalls?

Thanks

yes you can do that. I belive there is a standard notification you can use, and one of the options under NONELECTIVE is to select
must complete 'period of service' to be eligible for the nonelective contribution.
It saves the company money, especially if ees enter right away, don't defer and quit. fair is fair and the govt recognizes that.
see Q-10 of Notice 2000-3 for the full explanation

http://www.benefitslink.com/IRS/notice2000-3.shtml

A "pitfall" might be top heavy provisions. A participant eligible for 401k, still employed at year end, but who hasn't met non-elective eligibility, I think is required to receive a top heavy contribution.