It's possible, but it's probably not what you are interested in.
It's important to define what you mean exactly by "rules". Generally speaking, there are different server parameters that can be changed without needing to modify source code. You can find most of them here:
https://www.earthempires.com/express/serverdetails
If you wanted to pay for a custom server in which you set the rules as you want, I see no problem with that. But if you're looking to define new gameplay elements such as "Players who create countries are randomly assigned to clan tags." then that would require a change in source code which adds prohibitively expensive labor costs.