If you are working with very old mods (circa early 2000s), you might need ZModeler 1.07.
If you cannot export directly to OBJ, try exporting from ZModeler to a more common format like .3ds or .dae first. You can then use free tools like Zamzar or ImageToStl to finish the conversion to OBJ. A Piece on the "Digital Sculptor" z3d to obj converter
Free, full control over the export process. Cons: Requires technical tinkering; the add-on is unsupported and may crash on complex files. If you are working with very old mods
The most reliable way to convert Z3D files is using itself. This ensures all geometry and material assignments are preserved correctly. z3d to obj converter