What are the stipulations of combining AD military service with federal service to receive a pension?

Your buy back will be 3 percent of your active pay and you will receive credit for the years of active service towards your FERS pension. You have two years interest free after you are employed to make the payment before you start paying interest on the amount owed. Normally, it makes sense to buy back Military time unless you are retired from Active Duty and then you have to determine whether you will make up the difference between what you would get with one retirement less the buy back vs two retirements where the FERS retirement only covers Civil Service time and your Military time is covered by your Military retirement. If you are at a large most, CPAC probably hosts a FERS information meeting quarterly. CPAC won't care if you are not already a Army Civilian when you attend.

Correct comments. I'll add to take a look at the long game. I paid $10K (pay was lower way back then) when this FERS thing just started. Otherwise it would have been 7% under CSRS. No choice given when I came on board. That one time $10K (half of the separation bucks) is now paying $13K/year and gradually increasing due to COLA. Show me any stock option fund that will do that. I did reserves in which that retirement doesn't affect Civil Service. I guess I'm one of those hated 4-way, and later on 5-way dippers. BTW, unless the laws change as you age, push Social Security out as long as you can if it is still banging out 7%+ a year. No stock fund guarantees that either.

Everyone under FERS has to buy back their military service to get credit for it with their pension. Other benefits such a leave year computation do not require payment.