Some attributes can be added by turning the Smart Edit feature off and adding the Key and value you want, however that normally applies to triggers and functional entities like multimanagers, relays and multisources. The attributes that are diplayed with an entity, are there because they are part of the .fgd file and the .fgd file is linked to the hl.dll. So unless you want to do some heavy coding, you have to stick with what you have.