[PowerfulWizard]

This is what my priorities would be:

1. Make sure you have a good debugger set up for any existing code. This is to answer the question of exactly how a function behaves, what are the meaning of parameters, etc. You know you're going to be dealing with undocumented functionality, so you need a way to quickly answer your own questions.

2. Someone at your organization is paying for this right? Ask them to pressure the supplier for one-on-one support to answer your questions regarding how to use it. You want someone on a video conference who knows what they're talking about so you're can explain things quickly and not have to write up detailed emails and wait for response.

3. You could try to document it yourself: Locate all the exports and create a list of them. Browse it for key names and concepts. Write down the purpose of each, and their relationships. There might be a lot of these items but it won't be infinite. Even if there are 500, if you do 20 a day you'll be done in 6 weeks and by that time you should have a pretty good picture of what's going on.