Fitting point for roof bars on a 2010 ex British Gas.

[Firedawg333]

Hi all,
I'm after fitting a third roof bar to my ex British Gas caddy but it only has the front and middle fitting point.
The third has the indent circle in the roof but no hole.
Is it a case of drilling through the roof and then trying to find the bit the roof bar screws into?
If so where can I find the bit that fits internally onto the roof?
Also I see the internal roof lining has an Allen key fitting so do they just unscrew to drop the lining?
Thanks
Mark