Is it possible to make a plugin dependent on another plugin? If so, how?

I want to make changes to another plugin without changing that plugin itself.

http://cs.elderscrolls.com/index.php/De-Isolation_Tutorial

All the information there is valid for Skyrim modding.

Thanks Thomas Kaira.

It looks like the CK doesn't allow me to use two ESPs as masters at the same time, though.

The engine doesn't mind ESP masters, but the CK won't allow it. You can make ESM'ified copies, with WryeSmash, of the ESPs that are to be masters, build a plugin in the CK, then, also with WryeSmash, point the result's masterlist to look for the original ESP versions and all will work out for your plugin with ESP master(s).