Aw, that's a pity. If you have them pre-fitted, the fasteners are all hidden under the internal lining.
I'm afraid you are definitely going to have to drill the roof now. The Activrail usually comes as 4 pieces about 400mm long which they fix to the roof front and back on both sides. I had my roof done as a special order with full length rails as I wanted to be able to space the Sherpas Combi-Translift cross bars at different distances to accommodate snowboards (760mm gap) and surfboards (about 1500mm gap). If you are unsure, try contacting Johnny at Dirty Weekender Adventure Vans in Worcester. He's a very helpful guy T: Tell him I sent you if you do.