Adding/Editing resource types
How feasible is it to add new zotero resource types (I don't know what they are called, examples of existing types would be "artwork, audio recording, bill, book, interview, journal article etc...")? How would this be possible? Is it possible to add to the fields that are already available for existing resource types? How would this be possible?
Making custom item types and fields—and data that references them—transferable, referenceable, localizable, and forward-compatible is, unfortunately, a very difficult problem. We're committed to addressing it, but it'll take a little time.
Why not use tags for organizing information about interlibrary loans?
Once you've added the tags you need and tagged the relevant items, you can then sort the tagged items using Advanced Search. You can also make search folders containing items with whichever tag or group of tags that would be useful for you.
More information about tags and Advanced Search can be found by watching the Screencast Tutorials:
http://www.zotero.org/documentation/screencast_tutorials
Custom types are a large can-of-worms.
This topic title seems to suggest that it is exactly about this can of worms.
In any case, my point was just to say that that there are really two separate issues here (custom types vs. support for graphs and datasets), and that one can ask for both.