I searched "loft" on here for days before I came across the term "mezzanine" being used on here.. Try searching that..
Weld or Drill to attach brackets to your metal frame. A lot of factors go into the decision on what the best method will be for you: how large of a loft area do you need, how much weight do you need to support, how are you going to access the loft, how structurally sound is the existing structure/will it be strong enough to support the loft, etc.???