Yes it is possible to create and share workflows using JBPM. If you are looking for a full blown portal and also use JBPM you might be intrested in jbossPortal. It supports work flows using JBPM. Have a look at http://docs.jboss.org/jbportal/v2.6.4/referenceGuide/html/workflow.html.
What you are asking for is exactly what BPM is supposed to do. However this question is not simple indeed and JBPM only provides you with the technical stuff. Here is somme consideration to take into account, hope this will help you
1/ concerning the question of editing a process,
- there is the signavio solution allowing you to let people do it through web.
- there is the JPDL plugin for eclipse and more recently the BPMN2 one from Koen (see the forum)
2/ But editing a process generally also means parameters. This is basically done though forms and there is work to do there for type genericity.
3/ In aditions you will have to decide to maintain a business database (as a computer science folk) or use process variables extensivelly. To me a db is better and genericity is more difficult again.