No it's totally possible to use your own VCS and sync changes to another VCS. I don't know which tool you'd prefer but I used a git repository for my own work and synched it to a TFVC repository until my company switched to git. It sucked, because TFVC sucks but it's totally doable

no, its absolutely possible, as demonstrated by the numerous repositories that switched from cvs and svn to git. what you describe is a lack of desire to switch to another tool by your coworkers because you've been unable to make an adequate case for doing so.