Since version 7.0 it is possible to add extension packages to Game Maker. Extension packages extend the functionality of Game Maker by adding additional action libraries, functions and/or constants.
Additional extension packages for Game Maker can be found by pressing the Extensions Link in the menu on the left.
Creating extension packages is relatively easy, although you will need to understand something about programming. Extension packages are based on (a combination of) action libraries, DLL files, GML script files, and other files. To create an extension package you will need a special package that you can download here:
Extension Maker (zip, 800K)
The package contains two programs. Library Maker is a program meant for creating action libraries. Extension Maker it the program with which you make the actual extension packages. Please carefully read the enclosed readme file and the help files. It will be impossible to effectively use the programs without that.
Note that these programs are not supported in any way. They are provided AS IS. Please read the license agreement that comes with the programs. Also note that Game Maker does not do extensive error checking on the extension packages. So better be sure they are correct before using them.