According to this, XC40 only has 1 speaker on the front doors. There is no place for additional speakers! On the other hand it has two speakers on the rear door.
If you do not have the roof speakers already, most likely; you won't have the wiring too.
I don't want to discourage you but it won't be easy to find used parts because not many people has XC40 with H&K or B&W and they are not the same speakers as 60 and 90 series.
So, my advise to you start from the basics before even trying to finds the parts.
From the Orbit, enable the SUB and just connect a speaker to the AMP. "You can push wires into terminal holes" and check if you even get an output from your existing AMP (AUD) with its current config software.
Not sure if your basic AUD "No SUB" has center (Dash) and roof outputs but you can test same way as the SUB.
If you do not have the roof speakers already, most likely; you won't have the wiring too.
I don't want to discourage you but it won't be easy to find used parts because not many people has XC40 with H&K or B&W and they are not the same speakers as 60 and 90 series.
So, my advise to you start from the basics before even trying to finds the parts.
From the Orbit, enable the SUB and just connect a speaker to the AMP. "You can push wires into terminal holes" and check if you even get an output from your existing AMP (AUD) with its current config software.
Not sure if your basic AUD "No SUB" has center (Dash) and roof outputs but you can test same way as the SUB.