Not an easy thing if you already have a fATPL. Firstly you need a green card to work, you probably won't be eligible for a training visa because you're too qualified. Secondly, you'll need 250 hours for a commercial, you'll probably need to do another 300nm cross country, as the FAA require you to go 250nm away, then you'll need the IR and CFI tickets both of which require pretty thorough knowledge of FAR/AIM. You might be able to get a visa if you work at an EASA school in the US, which means an FI rating as well.