Can Capella be used as a requirements management tool itself?
I don´t think so. There are no “Management”-capabilities -as versioning etc.- implemented (yet).
The way to handle requirements at this moment seems to keep them in a Re-managment tool like doors, export to ReqIf, import with the Req-viewpoint, assing the reqs to Capella-elements.
Of course it would be possible to write a viewpoint to do all these things but its not the core-competence of capella to provide this.