I know how to add a DME arc into the Nav display, by entering it into the FMC Fix page. It shows up as a dotted green arc. Is it possible to add the DME arc to the active route so that it becomes purple and can be tracked in LNAV?

You forgot to mention type aircraft and signed name.I'm pretty sure the only way to add an arc is by inserting waypoints at bearing/distance, someone posted an example here a while back.

If he can do it on the FIX page, we know it isn't the Mad Dog!Paul