Form 8606 is only to be generated if you made a nondeductible traditional IRA contribution, made a distribution from a traditional IRA and you had basis in nondeductible traditional IRA contributions, did a Roth conversion, or made a nonqualified distribution from a Roth IRA. A rollover to an IRA from a 401(k) is none of these even if the rollover included after-tax funds. The rollover of after-tax funds to a traditional IRA does add to your basis in nondeductible traditional IRA contributions, but it is not reportable until you are required to file Form 8606.
When you do make a nondeductible traditional IRA contribution or a distribution from your traditional IRAs, requiring that your tax return include Form 8606, you'll report the after-tax basis that your traditional IRAs acquired from the 401(k) as an adjustment on Form 8606 line 2. This adjustment will require you to provide explanation. Using TurboTax, you would indicate that you have basis in nondeductible traditional IRA contributions, then click the EasyGuide button. TurboTax will then provide an option to enter basis rolled over from a 401(k).
