Right now you can not reference files, but you can create a field for uploading svg files. (Paramatric Model Maker V0.8.0 support uploading files!)
Users should then download you file and upload it into the form.
Right now it is not possible to modify the parameters in the 3mf file (it has been asked, so maybe in a future version it is possible).
In my case, usuarlly I add a section if there is any important setting to be set, and also add a print profile with this custom parameters, so users can download the profile and then drag and drop their design over the build plate and use the same parameters.