You can puchase seperate lamp brackets and mount them on your helmet.
Caving supplies supply CS Brackets (£1.30) or Petzl Brackets (£2.40), along with bolt kits (£0.83) or rivet kits (£0.83). They also offer a fitting service at £1.95.
With additional brackets, you could then cable-tie non-mount lights to them.
Just remember though, don't go too mad with brackets as you could weaken the external shell strength of the helmet.