My understanding is that you may manually construct an arrival / departure proceedure in the FMS / INS if either of the following are satisfied:
1 It is an overlay proceedure where waypoints are also specified as range and bearing from a conventional navaid. (Backed up on the needles.)
2 The proceedure is not designated PRNAV / RNP1. Or if it is, it must also state on the plate that is available in BRNAV.
Im more than willing to be put right though, the whole introduction of RNAV SIDs & STARS has lacked coherency and is still a little confusing.