I just happened to post about this exact thing in my release thread, so I'll paste it here again:
>I don't plan to add logging in to specific boorus because it takes a load of time to write them one by one, but future rewrites of the downloader engine will permit more and more complicated customisation, including eventually a 'this site can support a login, and here's all the FORM data for that', so that anyone with a bit of html knowledge will be able to add this support to any site in hydrus in a few minutes.
So there is no way at the moment, unfortunately, but I would like to support it in the future. A soonish planned rewrite of the networking engine will add more per-domain recognition and support, so perhaps then I could add some options for 'whenever on this website, add these cookies', and then people could copy their browsers' login cookies over, but that might be a bit technically complicated.