Go under the Format Editor menu and choose Expose Formats. From there a window will pop up showing all the current formats. Pick the one that you want to modify and double-click it to edit. From there you can Save As... and name it however you like.
As for the impo question, PSM can't create impositions. You'd have to send it an imposed file. If you are running film, there are some options available to maximize your film usage, but the controls are pretty limited. In the Output pane look under Expose Params and you'll see Layout, which has a few options under it. There are also more settings available directly to the right under the Params. button.
Hope that helps! I spent a long time (10 years) using PSM and recently switched to Prinergy. Me likey!