Well, in theory, you can't..
BeoLab 2500 is made with a basic Powerlink input, without L/R setup, it has no phono inputs, the only solution that might work (without being 100% sure) would be to use the latest phono/powerlink cables sold for the new ICE powered BeoLabs (3/9/4), that work in mono mode: each cable connects to the phono plug at one end, and goes to the Powerlink plug at the other end.
Maybe you can borrow the cables & check at home, or simply go with your Serenata to your B&O dealer & try it with a pair of Lab 2500, if they still have some on display. It sure will work with the new Labs , I had this setup with my Lab 3 when I bought my Serenata.
Reunion Island is greeting you!