As you may have noticed, FTB refocuses about four times while launching the selected pack. For example, if I launch it and then come to Arqade, it refocuses when it checks for an update for the pack. then, it refocuses during the Forge initialization. Then it refocuses yet again when the window for the pack appears. How can you prevent this from happening?
Put simply, No you cannot.
Your only option would be grabbing the FTB launcher code and compiling your own with workarounds to prevent it stealing focus.