I figured it out! The sleeve pieces came already attached and they looked like permanent pieces of the end piece. Well, a needle-nose plier and some brute strength (well, slight exaggeration :D) later, I got the piece off and now it will fit other pipes.
I read in the instructions that 6 sleeves were included but I assumed they were forgotten or something, heh. So now I'm all good to start quilting
Thanks for your help