@Siedlerchr Thanks for pointing out this importer, which I wasn't aware of.
However, the only importer that I can find is for RePEc NEP, which stands for "RePEc New Economic Papers". That's a weekly e-mail newsletter with a list of new papers on a chosen topic.
I presume the idea is to import the entire list from an email into JabRef. While this might be a useful tool by itself, it is not a substitute for a database inclusion.
In general, the issue is not about importing at all, because RePEc entries can be exported as BibTex files and then manually pasted into a JabRef bibliography. What I (and many others) wish for is a fetcher that automatises this task.