Dealing with nonstandard installs

[Watson Ladd]

Dear all,
PARI comes with optional packages. To install them, they need to be
untared on top of the installation Macports makes. I'd like to make
this part of the usual portfile for PARI, but there is too much magic
for me to figure out how to do this. Does anyone have a resource to
point to for helping me understand how Macports works under the hood
so I can make portfiles for ports that don't work via the usual
download, configure+install?
Sincerely,
Watson Ladd