@sven-v
As far as I know it's not possible to do what you want in MRA
But.... there is a workaround
Let's say you have a route with waypoints 1 to 6 and you want to set a winding route level between waypoint 3 and 4.
Proceed:
1.- Make a route A from 1 to 3
2.- Make a route B from 3 to 4 and set winding route level as you want. Expand that route to more points (dependint how long is). Let's say you expand to 40 waypoints
3.- Make a route C from 4 to 6
Now, join all the routes and the trip 3 to 4 will keep the route designed with the winding level you choosed