Yes, you can add new rollover money from your 2021 job into the IRA.
You DO have a traditional IRA (assuming your 401k was pre-tax). There's nothing different about a rollover IRA except the original source of the money, which doesn't matter for most purposes. (For some purposes, like bankruptcy, it may be advantageous to keep IRA funds from a workplace rollover separate from IRA funds from personal contributions. But for taxes, it doesn't matter. An IRA is an IRA.)
