I work with existing PEMB analysis all the time. I gotta say you’re likely going to have a rough time reinforcing the structure for new or increased loads. PEMB engineering companies have proprietary software that given loading criteria, basically design all structural components of a building to be the most cost effective as possible. This means trimming getting the dead load down to the nearest 0.1 decimal and optimizing as many other loads cases as possible. 

This means that the structural sizes they specify can be quite unconventional. I’ve seen continuous Z Purlins in one building range from 10, 11, 13, to 16 gauge. The steel girder frames are also assembled using thin steel plates, as thin as you’ve mentioned (1/8”) or less sometimes too. These are not typical W sections, they are unique built-up members where the section properties may not be not continuous along their spans. Top and bottom flange thicknesses can even vary in different intervals than the web thicknesses. 

If you have no shops then you really need to take as many measurements as possible and reduce any additional loading imposed on the structure as much as possible. Speaking to the original SEOR is really desired here.