What kind of data do you want to synchronize between the databases? logically the data needs to be separated and the relational data of your system needs to be in your own tables and the data of jbpm in its own tables.
For example If your system has its own business logic like managing registrations of some kind, where each registration has its own life cycle and data, then you could have your own relational database/shema and create the lifecycle-business process model of your registration in bpmn-jbpm5.
The user interfaces - human tasks can be part of the jbpm5, however the data from the human tasks can be stored in your database/schema but the bpm related data are from/for jbpm5 and they are automatically stored via jbpm5 persistence in jbpm5 database/schema. If you have variables that you want to use/manipulate etc in your business process then you can pass them to jbpm5 and retrieve them later or simply persist them from jbpm5 in your database via some kind of custom service task that you create.
The integration between the two systems can happen via the jbpm api or by using rest web services of jbpm (i.e. http://your-address/gwt-console-server/rs/server/resources).
We work in very complex projects of this nature, with information systems that have their own db etc. and they integrate nicely, to be honest you have so many choices and you are so flexible that you might go a little dizzy at first , but afterwards you'll see that everything fits nicely together.