You want to port the FreeBSD package manager to Linux so you can use Linux as a platform to build packages for FreeBSD? Am I understanding that correctly?
If so, this is almost certainly an
X-Y problem. Let's revisit your question: you're wondering if anyone has bothered to port
pkg(8) to Linux, because you'd like to use it on Linux to build FreeBSD packages. The answer is probably, "No, no-one has ported
pkg(8) to Linux for that or any other purpose." What someone quite likely
has done, however, is addressed whatever deficiency you think there is in building FreeBSD packages on a FreeBSD system. Clearly, the rudimentary tools for building packages using FreeBSD exist. On top of that, FreeBSD already has its own bulk package building and continuous integration systems. Not too surprising, since the package manager would be worthless without them.
So why would you go through the extra work for some dirty hack to get packages to build on a non-native platform? What are you
really trying to do? What do you
really expect to happen?