How to move from activity modeling to class modeling in UML diagrams? With respect to object oriented software engineering. Not: I am not asking for any reverse engineering tool.
I think you are missing what the goal of both this models are. An activity model only shows how something is achived. For example which inputs are needed, and where something happens with those. It describs a process, but doesn't take in account what technology is used. Wheres the class model is specific for object oriented languages, thus a technology. And with the class model you can't follow from the start to the end of a process, because it doesn't describe one. It only describes what you use to handle a process.
I hope this helps to clear how you have to approache this.