EDIT, sorry I misread some of your info.
Thank you Chris for your reply. I actually don't want the STL files to contain any material info. I just want the filenames to say the material name of the body (e.g. "CarbonSteel.STL"). Can this be done?
Have a look at the following example to save solid bodies into a new part : Save Solid Body to File Example (C#). Change the file extension to ".STL".
You can use IBody.GetMaterialIdName2 method to get the name of the material and use this info to save the body as material name.
Hope this helps.
I suggest you use the built-in SolidWorks material functionality. You just might want to change the favorites on a user's pc so only the materials that are used by the company show up.
Are you sure you want to create an add-in? It sounds like your experience is limited, in that case a macro would be easier to create.
Most of your questions can be answered by the SolidWorks API help, if you know what to look for. The most basic thing is to know the hierarchy, which means you first need an object variable to get the SolidWorks app, then the active model, then the part, then the bodies. You can then traverse the bodies, suppress the others if necessary and export the bodies using a save as function and a file name string created by you.