Is it possible to do The Pitt without a slave outfit?

I found the slave camp before I found The Pitt guy. I killed the slavers, freed the slaves, talked to The Pitt guy and didn't choose the right dialog options. Then went back to Megaton to do a whole lot of other things before returning to The Pitt guy, and only by now did I found out that I did choose the wrong dialog options. So, I need a slave outfit.

Problem is, I went there around level 11, I'm 25 by now. I don't have a save game from then nor do I intend to start the game all over again.

How can I still aquire a slave outfit? Or is it impossible now to play this quest?

Step one is to return to the train tunnel, which is near the north end of the map, northwest of Fort Constantine and a distance east of Raven Rock.

If you didn't take it and sell it, there's a slave outfit in the slave pen here, on a dead slave. It doesn't sound like you did this, but you might have and then didn't mention it in your question.

If you can't find a slave outfit, you should try entering the train tunnel anyhow - the quest page for Into the Pitt says that Werhner will take you to the Pitt even if you opt not to take a disguise.

I myself abandoned the disguise early on, and the DLC plays out roughly the same in this case.