Automatic Login from another secured site

I have a third party timesheet site for all employees that allows me to post HTML formatted entries (as in links), but not do any uploading. I have a Word Press site that I can post those documents on and is secured. Is there anyway to pass login information in the link from the timesheet site so we can avoid a double login (as well has having to maintain lists in two different places?

The documents are not for the general public, but not all that sensitive either. Any suggestions?