Hi Tanja,
Please have a look at this. You need to embed your custom component in BP_CONT by enhancing this standard component.
I can tell you briefly the steps :
1. Create Component Interface in your custom component for the window that you want to show in BP_CONT
2. Create Component Usage and choose this window which you added in component interface.
3. Add this usage in the main window of BP_CONT
Try the steps in the document and if you find any issue come back to us
http://scn.sap.com/docs/DOC-27478
Regards,
Devashish