Hi MD83FO,
can we use an fms waypoint in leu of an inop approach or departure navaid?
It depends:
On departure, if you can confidently climb above SSA/MSA without the benefit of the radio aid, then it's OK. Some departure charts will specify what aids need to be available, e.g. FMS + GNS, DME/DME.
When descending below MSA/SSA, you can't substitute an inoperative approach aid with a(n) fms waypoint, so you'll need the published approach aids to be working. There may be a GNSS app published (e.g. LGW).