If the cells aren't puffed then it would be impossible to 'squish' them at all. An un-puffed cell should be rock solid therefore incompressible.
If there is any 'squishiness' at all then it means the cells are pulled. If so it would be a very bad idea to apply any pressure at all because you risk bursting the pack which would be very bad news and could even cause a fire.
If the packs are at all 'squishy' then IMHO it's probably not worth the effort as they are clearly well past their best.