Login by URL?

Hello,

I want my users to be able to login automatically by URL-String.
The background: I’ve got another portal with registered users. I’m not able to connect the user database with wordpress’ user database, so I want to generate a link to wordpress in this portal containing the username and a password of the users. I’ve got the same user data on both sides; the passwords on the wordpress sides are generic. I know this is not secure, but it need not.

So my question is: is it possible to login to wordpress by and url, e.g. http://www.myblog.com/username=johndoe&password=xxxxxx ?