Editing /usr/share/init/tcsh/login will edit the settings tcsh defaults to for all users in your machine. If that's what you want, go ahead.
Otherwise, the traditional way would be to create a .tcshrc file in your home directory and put in the line
setenv MANPATH /new/path/to/man:$MANPATH
or something like that.
I suppose you could make a ~/Library/init/tcsh/login file and
source /usr/share/init/tcsh/login
in it and add whatever changes to the bottom.
Depends what you want to do...