Can you roll or direct transfer a Roth IRA into the Roth portion of a 401k which allows for both pretax and Roth contributions?

Only if the plan allows rollovers from IRA or Roth IRA. Some do and some don't.

I don't think that Roth IRAs can ever be rolled into a Roth 401(k). Roth 401(k)s can go to Roth 401(k)s OR Roth IRAs, but not the other way around.

Can't roll Roth IRA to Roth 401(k). My recollection is that there is nothing in the Code that permits it (not that any specific cite prohibits it).