a foreign customs
So is Christianity, unless Jesus was really called Przemysław and came from Rzeszów rather than Bet Lehem.
Christmas trees too, from Germany. Šw Mikołaj a Greek from what's now Turkey.
Every custom, every idea, every tradition has to start somewhere, only a few can truly be called home-grown.
And Halloween has been adopted wholeheartedly in Poland. As you may know, it's very big here now. Extremely big given all the celebrations, the merchandise, the special TV schedules. Bigger than in, say, the U.K. where Bonfire Night (and where I'm from, Mischievous Night) occurs at around the same time and also involves partying.