I don't think there's any way builtin to LR to add keywords while in the Map module.
You could try the Any Tag plugin, which would let you tag photos appearing in the filmstrip under the map. Currently, its commands appear under the Library menu (which doesn't appear in Map mode), but if you're interested, I could send you a trivial recipe that would cause them to appear under the File menu, letting you invoke the plugin in in Map mode.