I have done many similar integrations beetwen Liferay, as view layer, and existing business vertical application, such as eCommerce (Magento, Konakart, ...), CRM (SugarCRM, vTiger, ...), BPM (Activiti, Bonita, Intalio, ..), ECM ( Alfresco, Nuxeo, ..) and ERP (SAP by IDOC, Adempiere by WS API).
Always I have used middleware to integrate both decoupled layers, as middleware i use WSO2 ESB (I am official partner), Mule or/and Camel. For AuthN, AuthZ, SSO, Identity Management and Security integration I have used WSO2 Identity Server and external LDAP.