[xxpinballxx]

you need to know the voltage its going to use...the older flashes use a higher voltage than the newer ones and could fry the flash circuitry in the d90.....
there are hotshoe protectors that can help in this regards.....
i would get the specs on each or try a repitable camera shop before playing around and trying it.......there is a way of taping up the contacats in the flash too to get the same effect of the hotshoe protector but im not that familiar with it all....I just know if it were me I'd find a ringflash that is compatible so that im not in any danger of messing up my camera!
Good luck with it and hope it works out for you....

Just a quick google search and I see that some are stating that Lomo Ring Flashes are utilizing a much higher voltage than most of todays cameras......