There's confusion between the Toolbar and the ExplorerBar. OP's question (and mine) concerns the Explorer Bar. He/she wants to add a button to it that will run a command on the selected file. At least that's how I understand the question. I already have a button on the Toolbar that I can drag and drop a file onto to run a virus check on it. But it would be much better if I could select the file and click a button on the Explorer Bar.

By the way, I know this isn't an answer; it's a comment clarifying the question. I'm not allowed to comment (? :-[). Feel free to edit or delete it.