Read again what he said:
Imprisoning known and apprehended lawbreakers for the crime of illegally entering this country, in addition to whatever other punishment they receive for other laws that they have broken -- and then sending them back where they came from after their sentences have been served
He's talking about those already being jailed. Those we already pay for. Jail them longer for illegally entering the country and then kick them out.
If they know they'll get a longer sentence and get deported, they'll be less likely to come in the first place. Crack down on employers as well, but punish the lawbreakers, more.
If we could be assured that they will never enter this country again, I suggest we skip the jail time and deport them immediately. They managed to get in here because our government has blatantly broken the laws already in place. Law-abiding taxpayers rightfully have no obligation to foot the bill to keep these foreigners in our prisons.