There is a mechanism to convert after-tax portion of 401k contributions to ROTH IRA. Fidelity offers inplan conversions on a daily basis, so as soon as your paycheck contributions make it to your 401k that after-tax portion would automatically be trasferred to your ROTH IRA. Effectively there won't be any earnings on your contributions and your after-tax contribution just gets transferred to a ROTH IRA account.
https://www.fidelity.com/viewpoints/retirement/IRS-401k-rollover-guidance
