You'll have to duplicate the default texture, give it a different name, for example "asph_normal2.bmp", and then you'll need to create a file called "asph_normal2.bmp.cfg". Inside that cfg file, you put:
[moisture]
[surface] ... tells Omsi the kind of surface this texture has:
{type}
0: asphalt (standard)
1: concrete
2: cobllestone
3: dirt
4: grass
5: gravel
6: snow
7: deep snow
[surface]
1
Then, you can duplicate the spline, and assign that duplicate your modified texture.