Here's the dilemma. You have a traditional 401(k) that contains both after-tax and pre-tax dollars. You'd like to receive a distribution from the plan, convert only the after-tax dollars to a Roth IRA, and roll the pre-tax dollars into a traditional IRA. (By rolling over/converting only the after-tax dollars to a Roth IRA, you avoid paying any income tax on the conversion.)
For example, let's say your 401(k) plan account balance is $10,000, consisting of $8,000 of pre-tax dollars and $2,000 of after-tax dollars. Can you simply request a total distribution of $10,000, instructing the trustee to directly roll the $8,000 of pre-tax dollars to a traditional IRA and the remaining $2,000 of after-tax dollars to a Roth IRA?