Generally for calculating properties for built up sections or non standard sections, you can use Section Wizard which is also integrated with STAAD.Pro. These sections can be formed using a combination of standard sections and plates and Section Wizard would be able to calculate the properties for you and also export it to an external user table file which can be imported into STAAD.Pro.
As far as Cw calculation is concerned, design codes like AISC 360-05 or 10 provides values in the property tables but these are for standard shapes. AISC design guide 9, engineering text books like Formulas For Stress and Strain by Raymond J. Roark are some other documents that I can think of, which has some information on the Cw. I am not aware of tools that can be used for non standard shapes.
As far as Cw calculation is concerned, design codes like AISC 360-05 or 10 provides values in the property tables but these are for standard shapes. AISC design guide 9, engineering text books like Formulas For Stress and Strain by Raymond J. Roark are some other documents that I can think of, which has some information on the Cw. I am not aware of tools that can be used for non standard shapes.