It is possible to install a cat flap into the upvc panel, or even a double glazing sealed unit. However, the most effective option would be to install the glass pane from scratch using an already-made hole. Read the instructions that should be included with the kit for cat flaps, https://catflapdoubleglazing71284.ageeksblog.com/29943964/how-much-does-a-cat-flap-cost