You should be able to create solid models using the STL as a backdrop. Having not imported any STL files, but having created models using PDFs, I can guess they are a feature and can be put on a layer and blanked, leaving the placement constraints in place and not interfering with the solid models you build on them.
For STL files, it is my suspicion that converting it to a NURBS B-Rep by replacing the triangles with patches and then joining all the patches would result in a solid that is just too unwieldy to be useful for complex models, and not worth the effort for simple ones. A mid-complex solid has a few hundred sub-surfaces, but even a simple STL could have thousands, which isn't a problem for certain classes of calculations, calculations that are much different for NURBS.
The other thing that might be a problem is that NURBS evaluators seem to have trouble with 3 sided objects as they become degenerate at one vertex, leading to numerical instability. Basically, one needs to calculate the derivative/slope of the surface over the entire surface, and at a degenerate vertex there is no slope, it's just a point.
On STEP files for non-closed volumes, Creo can't make it solid, though you can, if you like, patch in the missing surfaces and merge them and then solidify. Otherwise, treat them as I suggested above - as a backdrop for building a solid model. When it comes to making a drawing, using the 'Hidden Lines for Surfaces' option under the view properties menu will allow the drawing engine perform the hidden line removal.
