It would depend on what you have in there already. Each Pim (shelf or box) has capacity for 64 TDM ports using card slots 0 to 7 so the maths tells us there should be 5 spare ports on the existing 55 cards and as the system has 64 card slots when full, we need to know some information. First, how many boxes are there? Second can you see any empty slots in any of the boxes but only counting any in positions 0 to 7 (the left most eight slots) of each box. It is hard to tell from the numbers you have given us as other cards can be installed in the same slots.

Worst case scenario would be that you could buy a second hand system quite cheaply, tie it to the existing one and with a bit of fancy programming make it seem as though they are pretty much the same system. I will however qualify that with the rider that some functionality would not be available between the two because getting the correct way of truly tieing them together is probably not possible due to licensing no longer being available.