Highlighting lexers are part of the [Scintilla](https://www.scintilla.org/) project, if an existing lexer isn't good enough then a new one must be added there first. You can look at using a different (existing) lexer in a [custom filetype](https://www.geany.org/manual/current/index.html#custom-filetypes).
You can add extensions to existing filetypes by editing `Menu->Tools->Configuration Files->filetype_extensions.conf`.