For the 401k, you just call up Schwab and they'll send you a form to fill out. Then mail it back to them. Takes a few weeks but it's just filling out a form. Make sure you tell them you want to convert your post-tax contributions to Roth.

There is no direct way to have post-tax go to a Roth - which would be logical.

You can also convert any other taxable contributions, such as A, B contributions, vacation forfeiture, etc to Roth. However, those will be taxable in the year you convert them.

I plan on converting all of my pretax contributions over to Roth after I retire and am in a lower tax bracket.

The two previous posts refer to IRAs; I'm specifically referring to converting 401k funds to Roth.