How exactly could you do this?
If you put a tariff on every overseas employee of a US based company, they'll simply create a subsidiary to employ them. If they try to get around that by tariffing employees of wholly owned subsidiaries they'll outsource them to an independent third party.
Some that employed more people overseas than in the US might just feel it is easier to pull up stakes and move their HQ elsewhere. You think Trump rages against Twitter and Facebook now, imagine if they re-domiciled in our greatest enemy, Canada?