The Add to Block command 
is available when editing a block in the context of other objects on the sheet. Use this command to select and add 2D elements to the block you are modifying.
The block is updated when you select the Accept button on the command bar.
Note:
The Add to Block command is available only when you have opened a block for contextual editing using the Edit (Block in Context) command. Nested blocks are not eligible for editing in context.