Q: How can I add files to an existing a ResourceData file
A: It's not possible to add files to an existing ResourceData file, rather you have to generate a new ResourceData file with all resources you want to add. Because the list of files to add can become big very quickly and you don't want to write the commandline everytime again, I recommend either to use a batch- or make-file.
Q: I want to use multiple ResourceData files, but get a linker-error
A: Actually you don't really need multiple ResourceData files, because you can add as much files as you want to one ResourceData file. However, sometimes you might want to split graphics from sounds or something. If you generate multiple sourcecode ResourceData files, you must adjust the arrayname, otherwise the linker spites out errors about multiple defined symbols. You can change the arrayname with --output-arrayname.