you can use chess board maker or if you want to make something like atomic mod you can learn a programming language and create your chess game with it or modify open source games from github.
Or if you don't want to learn programming you can modify chess games made from scratch(a visual programming language made for kids) or mblock(same) . I don't know any other way. Also it can be a challenge to make AI evaluating moves according to the rules you are created. Anyway at the end it is an outdated discussion
Is there a tool on this site that allows me to mod the chess moves? so subscribers could make mod versions of chess? And play chess variants also?
any support would be welcomed.