Sorry, it is not possible because the readme gives instructions precisely regarding the SkillGrid directory (and reflects the CD layout, which includes two directories).
In an AmigaOS environment, all that is needed to install the game is to extract the SkillGrid directory from SkillGrid-WM.lha and put it somewhere. If you don't have AmigaOS, there are tools for Windows and Linux (and most likely also other OSes) for .lha files. In particular, 7-Zip handles .lha files just like .zip files. I don't know how Amiberry works, so I can't give you instructions regarding that.