Do not set the MANPATH variable in OS X... it will break man!
These instructions will work, will not break anything, and you can automate with a script.
1) have the source file for the man page you want to install (i.e. document formatted with groff). If the source file is plain text, you can use a tool like txt2man to convert it, or you can learn the groff syntax and format it manually. I found a nice treatment on how to create properly formatted man pages here.
2) Name the file after the command that it documents, with a suffix of what section it belongs in. Most man pages are in man1, so if you have the git man source file, name it "git.1" if it isn't already properly named.
3) give the source file the correct permissions:
sudo chown root:admin git.1
sudo chmod 444 git.1
4) gunzip the source file (I use tar):
sudo tar -czf git.1.gz git.1
5) move the gunzipped file to the proper location (using -n argument to prevent writing over an existing man page)
sudo mv -n git.1.gz /usr/share/man/man1/
you're done.
man git