Can an expression parse a text file or the expression of another property in After Effects?
Yes, since expressions are basically JavaScript, you can load and evaluate external source files. Unfortunately the location needs to be hard-coded, since (AFAIK) expressions do not have access to the path of your AE project, so it's rather brittle. For instance:

try {
    myPath = "~/Desktop/source.js";
    $.evalFile(myPath);
} catch(err) {
    "MISSING";
}


Further explanation of how similar code works can be seen on the ProVideoCoalition site.

One further caveat is that the script is loaded and evaluated again for every frame that gets rendered. You might want to test how loading the script affects your render times.