Using SOAP/XML to get login data?

I’m interested in seeing if there’s a way to use SOAP/XML to pull in login data from a third party site.

Here’s what I want to do: I have a CMS running my site. I want users to be able to log into their account on the CMS and then have that login work automatically when they go to wordpress (installed on a different server) to do a blog posting.

Possible? My CMS exports SOAP/XML data with the login info.

Thanks!