Have you *any* access to the non-switch end of the conduit? If so, if the existing cables aren't too tightly packed, you can disconnect one of them, securely (but neatly) attach a single earth wire (single-insulated green-n-yellow conduit-rated) to the disconnected end, use the existing cable as a draw wire to pull up the new earthwire, then (here's the cunning bit) pull up another conduit's length plus-bit-for-luck of the earth wire, reattach the existing now-middle of the earth wire to the existing twin cable, and draw it back *down* the conduit with the earth wire. Result: existing cables + earth wire in conduit.
Of course, you can get boring with using polypropelene string as the 'intermediate' drawwire (and can then afford to leave a length in situ for later draws, if there's any chance of further additions) - but using the same wire off the same reel gives you that nice 'minimalist' feeling ;-)
HTH - Stefek