The simpler parts that we use are created using Product List-controlled patterns, such as 2523 for elbows and 2047 for tees, and so on. We only use converted 3D geometry for very complex parts such as valves that have connections all over them at weird angles, or in cases where the client wants to see super-detailed output.
For cases that need high-res geometry for some reason, as you describe, then yes, to your point, you CAN take geometry from Revit, save it as a .dwg, and then convert that part, in Fabrication, to an .itm file. However, that would result in a pretty big file. I've found it better to create the appropriate geometry within the AutoCAD/Fabrication environment first (or import it from a .stp file supplied by a manufacturer), and then convert it to an .itm, all within the Fabrication environment. At least for me this typically results in a much smaller file (fewer polygons).
(In certain cases you can also use Inventor to reduce geometry file sizes as well, before converting them.)
However, Jona also brings up a separate point, that you also allude to, that has thrown me off: converting 3D geometry to an .itm file results in Fabrication packing that into a pattern 928, which--to my knowledge--is not and has never been compatible with the Revit environment. Depending on the year you're working in, you'd have to then user Fabrication to do an extra step to repackage/convert that geometry from 928 to either 1175 or 2199 which, again, depending on the year, handle Product Listed data differently.
