Yes, you can add custom instructions in the directions panel of the route planner. See the following How-To Guide for more information: How To Add Additional Notes Or Directions To A Route
Does plotaroute have custom course points ?
THe Garmin edge cycling units will auto reroute the plotted path if the route does not play nicely with the underlying garmin map. So to bypass this auto-routing you can disable the recalculate and turn guidance basically just blindly following the magenta route. Therefore there's no turn guidance at all in this mode.
If i could place a custom cue/custom waypoint/custom course point with a custom message taht'll pop up when it's reached will help tons with the navigation. Example - the custom waypoint "Turn left at red post office"......etc.