It depends on where the buttons should appear. For the starting form of a process, you can specify the button label in the Process Modeler by going to the message start event's settings.
For user tasks, you can specify the action and label for those buttons and, using gateways, can determine what happens when a particular button is clicked.
If you want to add buttons on your form, you'll want to use the Custom HTML fields and enter something like this:
<input value="Done" type="button">
Of course, you'll need to use JavaScript if you want a certain behavior to happen when the button is clicked.
If you want to cancel a currently running instance of a process, you can do this from the Results page for that process by selecting the instance and clicking Stop. After you've stopped the instance, it will be canceled and you can selected it again to delete it.
I might not fully understand what you're asking. Currently, the only way to cancel a process is to do what I just described on the Results page.