I am new to TM mods and trackmaking, and i was wondering if there is possible to make for example a 30 degree curve ?
I have tought about modifying ground textures, and "paint" a curve on the ground, anyone who can confirme or deny if this is possible in any way ?

No, this is not possible.
Most trackbuilders just make some S-curves to simulate this

If you look at Stadium from high up, you'll see the TrackMania logo mowed in the grass a few times.
It's a texture that is stretched to cover a lot of ground, so you could consider it low resolution.
You can change it into something else though, and I think that's what "The Elvenpath" track/mod does to have a path on the ground.
Since the TM logo repeats, the path probably repeats as well, but the track author hid the unused parts by blocks.
It's been a while since I played, but I think it uses MediaTracker to do something with the camera so you basically can't see where you're going if you deviate from that path.
There is a video to let it make more sense than my description.

Maybe you can figure something out from that track, and maybe you can contact the author for some pointers.