I've found some Gravitrino balls, and they're taking up a lot of inventory slots, can I stack these items at all?
Yes, but you initially loose 5 balls to start off with. If you craft an upgrade that requires 10 Gravitrino balls, and the destruct it, you get what looks like 1 Gravitrino ball back, but it's actually a 100 stack, containing 5 balls, now you can just stack the rest up with it.
This maybe considered a bug and might change in the future. I don't know if this works with any other high value item as I've only come across Gravitrino balls so far.