Govindan Nagappan
Ansys Employee

Shells should be imported as shell. Do you have "thick shells and beams" turned on? This would display the shells with thickness and could appear as solid in graphics screen. 

Also Solid and shell bodies have different icons in the Geometry tree. 

If thid doesnt help, can you provide more details about how you are importing. What keywords/commands are in your nastran file that defines the shell and solid