Assuming that there isn't hardcoding that gives you access to the First Cohorts once you have Rome - and presumably the Marius Reforms - why not simply mod them in?

As an idea, copy the hidden resource feature used for Sparta & the Spartan Hoplites and have the First Cohorts only buildable in Rome itself, which would make sense in a way since each Legion was presented with their Eagle by the Emperor. Now that I've thought of that, I'm thinking of doing the same with the Praetorians and Urbans to make them a little more special.