HKP is Mentor Graphic's file format, Hierarchical Keyword Parser. A set of HKP files can fully describe a board that can be exported from or imported to Expedition PCB (old name: Veribest). In theory other of their product would work with HKP files (such as PADS), but in practice the actual import/export could be tricky. Also, discussions on Mentor community forums show that the company started getting rid of the ASCII HKP format around 2010 in favor of an encrypted and/or binary file format, so an old version of the software or a special license with new versions might be needed.