Maybe. We'd have to figure out how to build it, and we'd have to rediscover the reasons for all of the seemingly crazy workarounds that were in the code as it currently is. In order to be able to add security patches etc. we'd first need to develop an understanding of the code, which while possible is not trivial. Even in the success stories of an open-source group adopting a previously proprietary codebase (e.g. OpenOffice, Mozilla), it takes a huge amount of work and years or decades to get the codebase to a level where it can attract outside contributors in the way that open-source-from-day-1 projects do. In fact I'm not sure I've ever seen such a project succeed without having a large base of paid contributors and outside funding to start with.