Premake is written almost entirely in Lua, the same dynamic language that you use while writing your project scripts. Because Lua is dynamic, you can easily replace functions, add new values, and generally run amok in the code to make things work the way you like.
We've structured (or are in the process of structuring, with the intention of being done before the 5.0 release) the code with this feature in mind, adopting coding conventions that make it easy to hook and override or extend Premake's functionality.
Use the Source!
Before you start hacking away, you should be comfortable browsing through the source code of Premake or the third-party module you wish to modify. You will need to be able to identify the Lua function that emits the markup or otherwise implements the feature you wish to change before you can hook into it.
If you haven't already, you should grab a source code package, or clone the code repository on GitHub to use as a reference.
Have a look at Overrides and Call Arrays to learn more about Premake's extensible coding conventions, and how you can leverage them to easily change its current behavior.
When you're ready, check out the documentation index for more customization topics like adding support for new actions and toolsets, and how to use modules to package your code up to share with others.