Adding to existing blocks

Is there a way to add an exact copy of geometry to an existing block to make it an instance?

For example, if I made 200 boxes and then decided to make them all blocks? or if I made 1 block and copied that block 199 times and then forgot that I made those blocks for some reason and made that exact geometry, could I make it an instance of an existing block definition?

much appreciated

No easy way I can think of.