Blaming the founders for slavery in what is now the United States is ludicrous. They did not invent the institution.
I understand that the practice predated the US of course.
Your argument would be (I think) analogous to the fact that IF I came went across the border into Mexico with Live ammo I would NOT be breaking the law because where I had come from it was legal. Fallacious of course as each time you cross a border the laws of that nation and no other are then imposed on you.
Thus when a person any person entered the US there status of free or slave would ONLY be determined by the law of the US what they had been before in time or place is not relevant.