first you need to know how to model it. If the model has been rounded by fillets/chamfers, youneed toguess where the original sharp edges were.
After importing the non-parametric model, check if they are in good shape (any missing features? more than one component?). if no, orient it to the best position (faces parallel to the reference planes)
Then start modeling, big features/surfaces come first. (similar techniqueto building a model based on 2d pictures).
Avoid making relations with the non-parametric model or using its edges/faces, otherwise the resultant model is not easyto edit in future.
if the model is simple in shape, built by pads, pockets, fillets, chamfers and some other simple features, i believe some softwares/pluginscan recognize them and rebuild theparttree with editable features.
however,if theimported model is complex, and stylish in shape (built by a unclear combination of surface operations), like agamepad& computer mouse, i don't think the original part tree can be rebulit. Technology of feature recognization still needsbigimprovement today.