You mean sharing a variable between multiple process-instances? This is not possible using activiti-variables, however, you can always use a static/singleton containing the values you want to share and hook it into the ExpressionManager of activiti (for example, by adding it to the "beans" map in processEngineConfiguration). If you do this, you can reference the "shared" variables from within expressions in your process-xml, e.g: "${globalVariables.get("someFlag" == true}".
If you want to have the global variables persisted, you'll have to handle this yourself. You can always leverage the existing transactions of activiti if required.
