Unless the employer contribution is larger than the permissible elective deferral, it would have been far easier if part of the employee contribution was Roth and the employer contribution was entirely traditional.
Roth employer contributions to your individual 401(k) are only permitted if your 401(k) plan agreement has been updated to permit such contributions.
The IRS has not yet provided guidance on how employers are to report Roth employer contributions to qualified retirement plans. However, if the one is to take a hint from how the IRS says Roth SEP contributions to be reported, one might guess that employer contributions to the designated Roth account in the 401(k) will need to be reported as a regular deductible employer contribution to the traditional 401(k) account followed by an In-plan Roth Rollover of that amount (without regard to any after-tax funds the employee might have in the traditional account), with the IRR reported on a Form 1099-R for the year in which the deposit to the designated Roth account occurs. The consequence of this would be that taxable income would be reduced in the year for which the contribution is made and is increased by the same amount in the year that contribution is deposited.
