You could try making a custom filetype [(read the fine manual)](https://www.geany.org/manual/current/index.html#custom-filetypes) if one of the existing highlighters is "good enough" when given your keyword list (maybe C), and if a parser is good enough (probably not). Otherwise a parser needs to be contributed to [Universal ctags](https://github.com/universal-ctags/ctags) where Geany gets its parsers from, and/or a highlighter contributed to [Lexilla](https://github.com/ScintillaOrg/lexilla) where Geany gets its highlighters from.