Define periodic BC on imported step files

Hy everyone, I am quite new to NG Solve. I am trying to solve stoke equation for a lubricating wedge. First, I solved this problem for a geometry, which I generated in NGSolve and it worked. For the next step, I would like to import a .step file and to solve the same equation. I can import the file and everything, but I cannot define a periodic boundary condition on the surface. The geometry of the imported .step file does not have the attribute geo.PeriodicSurface(…) Maybe somebody can help me and tell me, how I can define a periodic boundary condition on imported .step files. Many thanks to all of you and best wishes, Raphael