Currently (as version 5.14) Activiti has direct connection to
- Alfresco for document repository
- Drools for rule tasks
- LDAP for groups and users
- Mule for sending messages
- Camel for sending/receiving messages
To integrate any other external system you need to use Java Service Task, where you can use Java classes to delegate workflow to your external system. These Java classes can take variables from your workflow, can direct to one of its outgoing flows and of course you can use any capability of your external system.