It's a little hard to say with certainty, not seeing the innards of these self-contained lights. However, where there's a will there's a way. If you're patient and good with a soldering iron, you may be able to de-solder the wiring to the internal battery holder, remove that holder and then solder the wire to longer leads to a toggle switch, which will be connected to a separate battery (you shouldn't need a lot of battery power).

It would be cheaper to go with something like this--a flashing led wired to a battery case with switch. http://cgi.ebay.com/RED-FLASHING-5mm...in_0?hash=item
Better yet, if you want only one battery source and one center off toggle, buy one or more of these flashing leds.
http://cgi.ebay.com/FLASHING-LED-5mm...item335385d54b