I'd suggest two approaches, neither of which fully do what you're asking, but there's nothing I can do about that. 1) Export processes individually from within the app.
This will allow you to import these processes freely if you perhaps have a new install or want to move processes from one install to another. However, it's slow and you won't be able to export processes that have been deleted.
2) Query the database for the JSON models you're interested in maintaining.
Within the database, models (processes, forms, apps, etc - contents of the Models DB table) are all stored as JSON; in general, from what it seems, Activiti does not store anything in its database as XML - so you won't be able to query for process models in XML format. On the plus side, you will be able to query for all of them at once and maintain that information however you'd like. The downside is that this information really isn't something that's super useful to the average user, since you won't be able to import the JSON structure back into Activiti.
EDIT: For future readers; Oops, forgot about the REST API endpoints for exporting. Use that.