Afternoon. No. The Legend/Vista can't do this. The E20/30 is a bit better and will detect and count down to upcoming waypoints in Track mode but even so I don't think it prompts (bleep/light).
And you can add and name the waypoints direct in BikeHike, using Mapsource doesn't add anything to that other than its inbuilt 'snap' feature which is very useful for pinpointing junctions quickly.
If I understand correctly, you have Mapsource but no Garmin maps. Have you got your OSM maps displaying in Mapsource?
If not, that's your best next step.
If so, I'm not sure that you gain much from using BikeHike. It's so easy just to plot a point-to-point ('direct' or 'off road') Route on the map in Mapsource, and uploading to the Garmin is integrated here too.
OK, how about this for a workflow?
1. Generate and download your Track (or obtain it from Organiser, or whatever), and display it in Mapsource overlaid on your OSM map.
2. In 'Properties' colour it green, downsample if necessary, and name it something sensible.
3. Use the 'Waypoint' tool to lay down and name waypoints at all significant turns, and controls etc. Each name must be unique, and I advise not more than 6 characters.
4. In Edit/Preferences set 'Routing' to 'Direct'.
5. Use the 'Route' tool to draw the route, joining the dots from waypoint to waypoint. Avoid 'closing the circle' by using the same waypoint more than once. If its a circular Route (most cycle day-rides are), draw two routes, one out and the other back.
6. Check the route(s) doesn't exceed 250 points (and actually that's far too many for a single day ride - too many prompts just gets annoying). Name the route(s) something sensible.
7. Save your file (optional).
8. Connect your GPS, clear memory, and 'Transfer' Tracks, Routes and Waypoints (but NOT Maps).
Takes longer to write than to do!
On the road, you will get turn prompts (beeps, lights, messages) and you will see the Track as a green highlight on the screen.