There is several ways of creating your own @Formula commands. One easy way is to create a serverside scriptlibrary and create a function that is named @MyCommand there. Include this SSJS library into your XPage / Custom Control and you have added a custom @Formula.

function @Getfirstword(data){

   return @Left(data,” “)

}

And the second way is to create your own extension to the Domino designer as Ulrich Krause has done in this new openntf project.

http://www.openntf.org/internal/home.nsf/project.xsp?action=openDocument&name=SSJS%20Extension 

There is probably serveral more ways of adding new @ commands to XPages, if you know a way make a comment.