there is no need to upload jars to jbpm. It is based on maven so the requirement is to have a valid maven artifact that is available for maven to download - either in local repo (usually ~/.m2/repository) or in guvnor's one. To build kjar you could use this archetype that provides you with all things you need to get started. Then place your processes in src/main/resources and build it with maven (mvn clean install). Once that successfully completes use it's GAV on deployments panel in jbpm console.
I have been trying to do the same thing. I built everything that I thought was required in the jar, including kmodule.xml. Then I uploaded the jar to the Artifact Repository. Then I went to the Deployments page and selected the "New Deployment Unit" button. I put in the values for my project and used DEFAULT for the Kie Base Name and Kie Session Name. However, I always get an error like "Cannot find KieModule." Maybe I am missing an expected file from the project?
Maciej, you mentioned a Maven archetype that provides all the things you need to get the project started. What is the archetype? Can you please provide the name or a link?