How do I roll an old IRA into my current 401(k), and why would I?

If your employer's plan accepts incoming rollovers (most do), you can move a traditional IRA into your 401(k) via a direct trustee-to-trustee transfer. You cannot roll a Roth IRA into a 401(k), only pre-tax IRA money and only the pre-tax portion.

The main reason to do this reverse rollover is to clear pre-tax IRA balances out of the way so you can execute a clean backdoor Roth without triggering the pro-rata rule, which would otherwise tax part of your conversion. Other reasons include access to the Rule of 55, stronger ERISA creditor protection, and the ability to delay RMDs on that money if you are still working. Confirm your plan accepts rollovers and only pre-tax dollars move in.