There is already a Pause button but only when the "full" progress dialog is used. Go to Preferences - File Operations - Progress Indicators and turn on Show progress bar speed timer when copying from or to: and select the types of things you want to use the full progress dialog for, or just pick Any location.
This has been requested on the forums lots of times. GPSoftware seem to want to do it but have said it will require a lot of internal changes and other things have taken priority up to this point.
In the mean time you can integrate a tool like TeraCopy into Opus to get a copy queue if you need one badly. Fellow member Oblias has created Opus buttons for doing this: