Hi, welcome to the boards!!
Let me see if I get you here:
You have masterSiteA which has a database and its own website. You want to utilize the contents of that database for slaveSiteX? Is that right?
If so, then yes. All you'd need is the login information for the mySQL server. If the mySQL server is "localhost" for masterSiteA, then you'd have to input the IP address and use port 3306 (I think that's mySQL default) and connect that way.
There is another option in here too. If you find that you can't actually connect to that database, you can use a remote mySQL server which is dedicated to strictly mySQL service. So it will be an IP and port, and you can modify it as needed. That may not be the answer you're looking for, but it is an option.