Well, you can add your own drawers to the palette if you want by adding them to our code. The main problems you'll face are:
1) You'll need to either use Graphiti's tools to create and add nodes to the diagram, which is how we do it, OR program directly with the GEF API
2) Somehow the node you create this way will have to store its properties in the .activiti file, which I imagine will be extremely hard to do if you don't want to use our model and extend it for that node.
The way to go is therefore to customize and extend Activiti Designer. You can't do the things you want using CustomServiceTasks, but if you add stuff on to Designer, you should be able to accomplish it. If you want to keep up with new releases of Designer, though, you're going to have to re-add your customizations with each release.