PCB Reverse engineering in ALTIUM
AD in fact DOES have a CADSTAR importer, yet the quality of such imports is always pretty questionable - I wouldn't just import it an then work on that data.

Yet, there are some options to make sure your work matches the original work. We regularly do such work and some proven tasks which may aid you during the way are:


Try to get or create a netlist from the original design (which nets are connected to which pins). When you have finished your design, compare both netlists automatically (you'll need a tool for that). This will make sure that each pin is connected where it should be. The original netlist can probably be generated from within CADSTAR or from Altium's import.
Try to import the PCB and place your parts on the original positions of the design. This will make sure that the design matches the original design. If it's impossible to import the PCB, import the Gerber data and use PnP data to place your parts.