If you just want to embed login screens, you might be able to do that with Iframes. But if you actually want to automate the logins, then I do not know if it is possible, I've never tried. However, as no one else has replied, here are my thoughts...
One way of doing this depends on whether these sites are designed to take the login details as GET or POST variables. E.g. GET variables are added to the URL like:
Hopefully that will give you an idea.