thanks for the hints. A few comments on them:
The models are only let's say "semi-scripted". The rationale is as follows. For each group of packages (e.g. vertical round resistors, honrizontal box-shaped resistors, ...) I made a parametrized 3D model in FreeCAD (I use a spreadsheet to set the parameters). then I created Python scripts to generate the footprints, which ALSO output a Python script for each group of parametrized models. I then load the parametrized model and run the generated Python-script. For each footprint it sets the correct parameters into the Spreadsheet, recalculates the model and exports it (currently WRL+FCStd).
Currently I don't ahve time to recreate all the models as scripts (also some are fairly complex, e.g. common-mode chokes and diodes or LEDs). What I would be willing to do is open the existing FCStd-files, run a script that fuses them and then exports them to STEP+WRL with the better export of StepUP ... but for that I will have to run StepUP in an automated mode (thus the questions above) ... if I don't find a way to get it doen that way, I may hand-convert some of the models and the rest will stay as it is for now.