git: "overlay" of own remote-branch on official freebsd-ports repo

I do not know whether I'm right in this list, but since the subject is mutual so common
in development and GIT, I have the strong feeling I'm right here.

Im quite new to git, so apologizes for any inconvenience reading my question.

Using poudriere on 14-CURRENT to create a selection of packages also includes updating
the ports tree on a regular basis. I maintain some "special" ports not official part of
the FreeBSD ports tree and some other ports are part of those I'm supposed to maintain. I
keep personally track of the changes in a git repo of my own.

Now I'd like to "overlay" the official portas repo by that of mine to include changes.
With SVN, there was no problem to have local changes not overwritten by regular updates
of the ports tree as long as the specific port in question wasn't updated by the official
site. In git, this behaviour seems to have changed, any changes I made so far are gone
after poudriere is adviced to update the tree.

I'd like to ask how FreeBSD developers and maintainers do the trick. If there is an
official cookbook fpr maintainers (I haven't found it yet ...), please be so kind and
refer to it. Any advice is welcome.

Kind regards and thanks in advance,

O. Hartmann