It is possible to use web service API to integrate with WSO2 Identity Server. WSO2 IS has user-mgt,entitlement APIs.For any user-mgt action and check authorization action, we can call these APIs from any product. Sorry I couldn't find any tutorial describing this. Hope following blog post may helpful to you at some point, as it is recommend you to have external LDAP user store. It describes how to connect external LDAP with WSO2 Identity Server.
http://blog.facilelogin.com/2009/06/connecting-wso2-identity-server-20-to.html