Hello,
this is not supported out-of-the-box, e.g. there is no switch to flip to use an external Activiti server. As the Alfresco Activiti integration is rather intensive and affects some serious low level stuff, quite a lot of the features / functionality of the integration would not be possible with an external Activiti server.
Technically, it would be possible to integrate an external Activiti server as another (say: secondary) workflow engine in Alfresco - but you would need to implement the necessary interaction code yourself to both provide all the abstract functionality the Alfresco Workflow API requires and map the data as required.
Regards
Axel
