It's theoretically possible, but very hard.

Basically, find all classes that have "Git" or "Subversion" in their name, and all methods (Some places don't have separate classes, but a single class with 3 methods), and implement a similar class for your VCS.

None of the classes you'll be extending are "public api", and they might change without notice, breaking all your work. This kind of task isn't supported, (see T5447, only more so). so you'll need to be willing to invest some resources into learning the internals of Phabricator.

Without knowing anything about your in-house VCS, I can suggest trying to invest in bridging it to git or svn - that way you could use vanilla Phabricator (And other tools), and it will let your users use git too, which will probably make them happier. I suspect it will end up being simpler over-all, because of a smaller interface.
If you implement read-only git-compatible server, users and tools can "clone" that like a regular git repository.