I think the fastest way for you to get familiar with FreeSWITCH is to read the book: http://www.packtpub.com/freeswitch-1-2/book
The book is quite short, and it answers most of potential questions, and for the rest of questions you have the Wiki and the source code :)
to answer your question, yes, there's a number of mechanisms to look up external sources for user information and credentials. The simplest one is to use mod_xml_curl: it requests pieces of XML from an HTTP server, and you can build the service with whatever database backend you prefer.