1- Yes, in community version 6.2 it's possible to add your own methods both in drools-wb and kie-wb. In order to do that you have basically 2 options.
1.1: use the source tab.
(note: unless you have admin or developer role, the source tab will work in read only mode)
1.2: clone the git repository where your project is, modify the given files and push the chages to the server again. (see below)
2- The workbench works on top of a VFS, and current versions and future ones are delivered with a Git based VFS implementation. So basically what you see is that the workbench works with a git repository. Even if it's theoretically possible to provide some other VFS implementations it's not an easy task. Said that, you can't cut this GIT connectivity. But given that the workbench internally starts a GIT service you can allways clone a given git repository in the workbench and have access to the needed files. In fact you can excecute the most common git commands.
Here's an example of how you can clone a git repository in the workbench.
git clone ssh://admin@localhost:8001/uf-playground
Finally it's worth to say that Data Objects are tipically POJOs (with some annotations) intended to be used in the context of rules and processes for carrying data. We should avoid to add methods like "sendAnEmail()", "doComplexLogic(), etc.