That is actually a cool feature in google and it works with GMapToGPX feature which can be imported into maemo-mapper (Make sure to select "Track" & "Full").

But you can do something similar in maemo-mapper by:

1) Click and hold on the screen until a menu pops up, select "Location", select "Add Route Point"
2) Now click and hold on the screen at a different location of the route you want to travel and select "Location" and "Download Route to..."
3) At the pop-up screen select the radial button "Use End of Route" and click "OK"
4) Continue step 2 until your custom route is built.

I did the above to build alternate routes to work since the main route is routinely a highway parking lot!