Do you mean on the inside wall? There is already an air brick on the outside wall into the cavity and also a vent at the top of the side wall next to the wardrobe. Can you fit trickle vents to existing UPVC windows?
yes you can drill through the plastic and have them installed.The damp could be caused by either poor ventilation or also could be mortar on the wall ties in the cavity causing damp to bridge onto the internal wall. But see how you go with ventilation first.