You can only enable preconfigured options (which are defined somewhere under /usr/share/X11/xkb on my computer) using xmodmap.
You can create your own keyboard configuration using xkbcomp, but it’s a pain in the ass and not super well documented. I did that once but forgot all about it.
Edit: OK scratch that. I was thinking of setxkbmap, but I’m pretty sure there’s also a bunch of limitations to xmodmap. Sorry.