Don’t know about loading a GPX route but here is a workaround if you can’t find a way.
You can force Tom Tom to use a certain route rather than the one it calculates by using way points. Add your start and finish points for each day. Compare the Tom Tom route with the intended route and see where the first deviation is. Put a way point to force Tom Tom to follow your desired route. Repeat until the Tom Tom route is identical.
You can plan a whole journey in advance on your computer using the Tom Tom Home application rather than on the device and save it as an itinerary which you can then upload to your device.
You are basically treating each day as multi stage route to force Tom Tom to follow your desired route rather than the one it thinks is best. Its a little tedious but it works.