Frezzer plugs are missing. Can we put new ones on even though the origianls might be in the engine?

Do you mean someone knocked them into the engine while trying to replace them? If so, they can't go very far so you can usually fish them out. The worst would be they would migrate over to the water pump and cause it to lock up. That is very unlikely to happen.

Freeze plugs can corrode through and leak but they usually don't fall out on their own. One exception is when someone used a rubber plug that has a bolt to tighten it in place and that bolt corrodes away. When pressure builds up in the system those plugs will blow out. The proper fix is to install new soft metal ones. They're properly called "core plugs" because they close up the holes in the block, or "core" that are used to shake out the casting sand, but we commonly call them freeze plugs because they will push out if the coolant is too high a percentage of water and it freezes and expands when it gets real cold.