Normally, in order to modify source code and recompile it, you would need the source itself. According to the online store, there is a source license available. It also says "Please contact sales@unity3d.com for more information."

I'm sure if you were really persistent in your reverse-engineering efforts, you could probably back-hack the assembly of the binary with a de-compiler or some such and modify that to some extent, but that would be in clear violation of the EULA and really wouldn't be worth your time.