No it isn't the same and you shouldn't be using P38 to fill a hole. Body filler fills dents, it can't bridge a gap as it'll crack pretty quickly.
To repair a skirt properly you need to bridge the hole, ideally with a piece of the same plastic and a suitable adhesive or plastic welding. You then need an adhesion promoter. You fill the repair with a flexible filler, plastic prime, filler prime, then a colour code. Your local paint factors will have suitable materials or google Wayside adhesives. It may well be cheaper to buy a second hand skirt.
What size is the hole try and insert some metal guaze with some adhesive then pull back through the hole so that it beds up against the inside of the sill (bent wire to to do this), then when set add some filler on top.