Most contributions that go into your 401k plan or 403b plan are "pre-tax" monies. Some plans allow deferrals above the limits and are ear-marked as "after-tax" contributions.  Plan providers will be able to tell you if there are any after-tax monies in your plan. Those after- tax monies may be eligible to be converted to the Roth IRA.

Kiplingers.com explains:

Not all retirement plans allow after-tax contributions. But if yours is among those that do, this is a great way to keep some of your retirement savings growing tax free without paying the usual price of admission to convert to a Roth IRA. Normally, you must wait to switch jobs or retire before you can move money out of your employer-based retirement account. But some plans permit in-service distributions, allowing you to roll over some or all of your 401(k) money to an IRA once you reach age 59 1/2 .