PRJ files are used for various projects in many different software, so it is very hard to give a precise answer how to convert prj to act. The real difficulty will be to identify your particular .prj file, because projects are typically tied to the original software only. To make things worse, many different .act files exist, so there can be numerous possible prj to act conversions actually. Probably related to export of 3D content though.
Updated: July 18, 2023