These scripts and plugins are extensions to the modeling program. They are possible because the developer(s) of the modeling program made provision for them through what you call an API (applications programming interface). Now these tools can be of two types
(a) Scripts. Stored and run in text format itself, script instructions are evaluated/compiled by the scripting engine each time they are executed. Often use a language specific to the modeling program (e.g. MAXScript). Reverse engineering isn't so complex unless they are encrypted.
(b) Compiled plugins. These are developed using C++ or sometimes .NET languages and are typically implemented as a .dll. This describes my tool as well as the Zmodeler filters. The .dll contains mostly assembly instructions and data in binary format. If you don't have the original source code it's near impossible to reverse engineer.
In this case it may be easier to do it by analyzing the game files instead. Too bad those who've done it already already don't want to share their knowledge.