Currently you can split a stack into just 1 other stack of said quantity. You can work around it for stuff like ammo, fuel and such by multifitting ships and then multi repackage them. However this does not work for minerals, PI and other things.
My 2 suggestions would be:
Splitting stack and then into multiple stacks of x quantity for the entire stack or just 1 splitted stack.
Or for a more full version you can add UI in the window where you can say i want x stacks of y quantity or split the entire stack into stacks of y.
