The users guide provides a good example on how to handle this here: Activiti User Guide - JPA Support and here Activiti User Guide - JPA
Also, for an example, check out the test case here:
Activiti/JpaTest.java at activiti-6.0.0.Beta4 · Activiti/Activiti · GitHub
Just to make sure I pointed you in the right direction, I tested this in my Activiti Enterprise 1.6 environment.
1. JPA is already enabled (Hibernate is used for the Enterprise tables such as COMMENTS, DATA_SOURCE, TENANT etc) so no need to configure.
2. Create your JPA Entities under the com.activiti.domain (Scan path set in DatabaseConfiguration.java)
3. Create your JPA Repository classes under com.activiti.repository (Scan path set in DatabaseConfiguration.java)
4. Add any beans to manage the Entities in com.activiti.extension.bean
5. Create your table
Entities and Repository classes will automatically be picked up.