Hey @Fabian-W, welcome to Backstage! Great to have you here 
This functionality currently does not exist, but I actually created a wish that would do exactly that! You can find it here Set the starting step of a track
My workaround (which doesn’t cover all user cases) is to use multiple patterns. So if I wanted to loop from step 32 to 48, I’ll copy those steps and then paste them in another pattern.