[Frans]

You can add any disk to a volume group but there are some considerations you have to take in to account.

if you add disk that has mirror or raid or none at all and you lose one of the disks this will impact your vg. You have to decide how you want your system to react
Always use best possible layout and try to stick to this as a principle.
The vg is only as protected as you make it
If you have file system that is not important you can add disk that is not mirror or raided to that VG and make file systems from this.