How to extend an existing PAC proxy file

Our company provides a pretty long PAC file for using the various internal proxy servers.

To connect some remote systems I needed to set up my own ssh tunnel to connect to yet another proxy server.

To use that proxy automatically I took a copy of the official pac file and added my own entries in the middle. – That works fine, but feels quite ugly.

The pac file should be simple javascript. Is it possible to build a new small pac file, that checks if my proxy should be used and otherwise simply fetches and uses the official pac file?

(That way the local script should be much smaller and always include any potential changes on the official file.)