Is it possible to add a new fleetaction and a corresponding button in the fleetscreen?  

Hey tygart!

It is possible to modify the existing actions or add new "variations", but not entirely new behaviours: these are done in code.

Most of what the actions do however, such as applying effects on the fleets/systems, can be modified by looking at the following files (in the game's Public folder), and overriding their entries (duplicate, then modify) with a mod:

EntityActions[*].xml: where the gameplay happens. You'll see "definitions" of each type, and you can add new ones (definitions, not types).

GuiElements[*Actions].xml: the link between gameplay entries and text/icons. One EntityActionGuiElement must be created for each ActionDefinition, or it won't display properly.

GuiElements[FleetsScreen].xml: the link between gameplay entries and buttons in the fleet action panel. If you add a new action, you have to add it to this list with the right ButtonType for it to appear.

SimulationDescriptors[*].xml: the effects applied by the actions. If you're not familiar with how these work, I'd suggest looking into the modding tutorials.

There is a mod on the workshop which modifies the existing Expedition action (it removes the movement point cost!),
you can find it here: http://steamcommunity.com/sharedfiles/filedetails/?id=1174035814

Thanks for the information. It was very helpful, but I was unable to create a duplicate button.  I kept running into the same error as the mod loaded like"Could not retrieve EntityAction 'newDefinitionFleetAction'".  I was able to modify the orignal button to look any other button with EntityActionGuiElement, and I used a simple simulation descriptor to modify the fleets vision range just so I knew it was working.  The problem seemed to stem from adding the new action with the original button type to the FleetsScreen. I'm stumped.  Anyone else try to duplicate a fleet action button?