You can add Cards to a Board in bulk from a search in your Project Library. 

Hit the bookcase icon on the far left of your screen to access the Project Library. 


Select "Cards" and filter your search by card category, fields, or type a keyword.

When you have all the cards you wish to add, select onethen hit the double ticks that appear in the bulk actions toolbar to select all.

Choose the "Add to Board" option and select the Board from the dropdown menu and save to add.