How can I add more objects to a block?

I forgot to use block editor.
How can I some existing linework to a predefined block?

My only idea:

Explode the block, purge the drawing, recreate the block.
I get asked to redefine the block, however, I cannot resave the block as
it still exists elsewhere in the drawing.

I couldn't find anything in the help menu. I was looking under "redefine".