you are welcome to provide a new fulltext fetcher.
However, as each journal site is different there is often no generic solution for all kind of portals.
Currently JabRef tries to find the fulltext by looking at different sites;
JabRef’s fetcher are either fetcher for metadata, fulltext, or both.
You can find all fetchers and their implementations here
JabRef also supports direct download. Just click on the Download Icon next to the Search Icon and you can enter a direct URL to a pdf for example.