To find out if your employer's plan allows in-service distributions for after-tax contributions you need to get your hands on the "Summary Plan Description." It should be clearly laid out in there. Then read other threads on this site about how to do the Megabackdoor Roth.
I do my contributions in this order:
Max out HSA ($6750)
Max out pre-tax 401K ($18,000)
Make Roth IRA contribution ($5500)
Make after-tax 401K contributions, then rollover to Megabackdoor Roth ($10,000)