SOAP calls are not currently supported with Connection although a full REST API implementation is slated for the 7.1(3) release I believe (no, I don't have a release date for it).
In the meantime you can do it in the same way you do with Unity - via ODBC. Connection 7 offers a remote database proxy access service which is disabled by default but you can turn it on and create an account you authenticate with that has remote administration rights.
The place to start is to setup and configure the CUDLI (connection's remote database explorer tool) application - the help file explains how to turn on and configure remote ODBC access for the tool and you will also need to install the Informix ODBC client drivers to make it work (links are on the home page):
http://www.ciscounitytools.com/App_CUDLE_LL.htm
once you get that connected and working you're good to go doing it on your own. Just setup your ODBC connection string to look like this:
Driver=IBM INFORMIX ODBC DRIVER;Host=;Server=ciscounity;Service=20532;Protocol=onsoctcp;Database=UnityDirDB;UID=;PWD=;DB_LOCALE=en_us.utf8
and you're off to the races. You'll find the DB structure somewhat similiar to Unity's but it has more tables (due to strict FK constraint model). Short version is you'd look for users in the vw_subscriber view - should do what you want (assuming this is what you were doing in Unity - if not let me know and we can look at how it would need to be different for Connection).
