The man page says "ckbcomp - compile a XKB keyboard description to a console font.", or that it converts an X keymap to a console one (loadkeys format).
Since the busybox kmaps are essentially the loadkeys format in a more compact form, yes, it should be possible to convert from them to whatever grub uses.
Though there's a whole lot of wheels here from a quick look
why even use the X keymaps, when there's a ton of them for the console? [retoric question]