How do add a column to an existing lookup while the (+) icon doesn't appear?

Hello Samiha, 

You cannot add any new columns here as it's a base object and located in a base locked system package. 



If you want to simply add more options to the account Industries lookup, you can do it directly in the Lookup section (System Designer - Lookups - Industries) :

Hope it clarifies!

Best regards,

Anastasiia

You need to create a replacing object and select "AccountIndustry" as the parent. Then you can add new columns to the lookup object. 

Ryan