Portal algorithm

Hi everyone! I''m planning to create a simple 3D-engine with portal support. The thing is that no editor I found support this. The options are to make the levels in an existing editor with documented file format and then create the portals with some kind of algorithm. I''ve given this some thought, but couldn''t come up with any good sollusion to the problem. Does anyone know how to approach this or have an article describing some clues? Thanx in advance!