Import is a big problem, because all systems are different, don’t support same fields/parameters/languages, database structure is different, export formats also. So there is no universal solution for this. I will try to help you with building import bridge script here. (SQL/PHP/XML/CodeIgniter Skills are required).
Lets say that you already have your data in some PHP array or object (this fully depends on your custom data).
Now you want create bridge script that will import this data into real estate script database.