New steel section in staad

Hi

My name is Rafael B from Mexico.

Im modeling a steel truss in Staad 20.07  11 33, I choose from the Steel Database a double angle. 

The section that i choose, after code checking fails by a small %.

In stead of choosing another section, can I add to the existing one a cover plate. (Im notsure thats is how it is named)

I mean, a plate on top of the double angles to increase area and inertia.

Is this possible?

Could anyone help me

Thanks