What's a "distribution" in your context?
This word can mean a lot of things. In a software context, a "distribution" could be any format for distributing software, like e.g. some binary package, or even a tarball as a "source distribution".
As others already stated, it seems you have "Linux distributions" in mind. Those are distributions of a collection of software that's somewhat integrated to work well together. Well, FreeBSD package repositories are a distribution (one of third-party software to work on FreeBSD) in that sense, and the ports are the technical foundation to create it. There are tools to create your own package repositories from ports using different build-time options, so in some sense, you already created your own distribution doing that. Of course, that's NOT a "BSD distribution", just one of software for FreeBSD.
You might be confused by the name BSD (Berkeley Software Distribution) containing the word distribution. That's just for historic reasons, BSD started as an ever-growing collection of software (distributed as source code) improving an existing Unix system. But eventually, it developed into a stand-alone operating system.
Linux is kind of an alien because it is not an OS, just a kernel, and typically combined with some GNU software (there are other options, see e.g. Android...) to form a complete OS. As a consequence, you can't use it without a distribution containing at least this software. Another consequence is there's never a clear definition of what is part of the OS and what isn't, these Linux distributions just place all the software they have to offer in huge repos.
This model just doesn't match for a BSD system. In FreeBSD's src
repository, you have the whole OS. There are quite some build-time options as well, so you could create a custom build, and even create installation media from it (everything needed for that is part of the system) ... distributing the result would be, kind of, a "BSD distribution". Once you start modifying actual code, the term "fork" would be more appropriate.
After understanding all this, what exactly are you trying to achieve?