Does anyone have a process for prepooling cryo using saline? If so how do you attach the saline?

Method depends on if you have a sterile docking device or not. Would this be for transfusion or at a donor center?

It would be at a donor center.

To the best of my knowledge, there are no commercially available closed-system processes for adding saline during donor center manufacture of pre-pooled cryo as saline is only available for purchase in bags (and maybe vials depending on country). BioPharm would add saline through a 0.2 micron sterilizing grade filter and consider it a closed process but these filters are not commonly used in the component lab. They are typically found in Apheresis equipment disposables where the "fluids" do not come pre-attached.