Q:
I'd like to know if I have to add an iDR-OUT for output expansion is it mandatory to add an iDR-IN as well to the iDR-8 processor?

A:
It is not necessary to add an iDR-IN when you add an iDR-OUT.
The expanders work as independent additional modules, so an 8 in - 16 out system is perfectly viable, as is a 16-in, 8-out system.