There is no additional tax from rolling a Traditional IRA into a Traditional 401(k). Both are pre-tax retirement accounts.
You would have tax due if you moved money from a traditional tax-deferred retirement account to a Roth because a Roth is an after-tax account, which is why retirement withdrawals from Roth's are tax free.
For more info see What's the difference between a Traditional IRA and a Roth?
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