Hi @Anonymous,
I'm not entirely clear what you need to convert. Is it that you have some old designs in pre-V6 Eagle format, and you want them in the XML format? If so, it should just be a case of loading them into V8 and saving again. You may hit some issues with inconsistencies between board and schematic, though, as V5 and earlier were a bit slacker on some tests than V7 on. I've hit issues where V4 allowed a BAS16 diode to be linked to diodes.lbr on the board but semicon-smd-ipc.lbr in the schematic, and was quite happy with it. V8 objects to that but it's fairly easy to fix.
As to whether you "need a library", if I understand your question right, then no, you don't. The information from the libraries that you need is embedded in the .sch and .brd files already.
If you want to make use of the Fusion 360 integration, then you will need to create a "managed library", but that can be started by exporting a project library and converting that to be "managed". I've not made use of this myself, though, so I'll leave it to others to comment further.
Regards,
Rob
