As stated, some people do make after-tax contributions to a 401k that are in a regular account and not a Roth account. The rules for doing a Roth conversion are different with a 401k than for an IRA. With an IRA, you follow the pro-rata rule, but with a 401k, the plan is supposed to separate the pre-tax and after tax amounts. (To the best of my recollection.)
Of course, it's much easier if you put your pre-tax 401k money in a regular account and your after-tax money in a designated Roth account within the 401k, but some plans might not offer that.
