argiro_ce:
If you are trying to transform a library part (object, door, etc) into lines and fills, there is a command for that:
Explode into Current View. If you are using the Standard Profile 14 Work Environment, then the command is under Edit > Reshape. The placed instance of the library part will be transformed into lines and fills that can be edited as any 2D element, but will have no connection to the 3D model, and cease to be a library part.
For more information with images, go to Help > ArchiCAD Help, when your browser opens click on the "Search" tab, and input "Explode into Current View" in the search box, in the results list "Explode into Current View" will lead to detailed information.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David