It should work but you'd have to make a solution of whatever additive, Calcium, Copper Barium, depending on which color you want, most salts will go solute in water but I'm not sure about lighter fluid, fireworks deal primarily in solids as the salts can be unstable if not mixed with other solids. Barium, Sodium, Calcium, Strontium are all allkali metals and alkaline earth metals which have a tendency to be unstable unless made to be stable. Pure sodium, for example will explode in water, treating these elements to make them more stable can make the insoluble in water and it would have to be done in in an alcohol, while this would burn in a zippo (Not recomended by the company) putting them into solution may take away and color changing effects they may have so even the ones that should go into solution in water either may not go into it in lighter fluid or may not give the desired effect and would be a waste of time. I'm not entirely sure, what would happen really, it sounds like a lot more trouble than it may or may not be worth.
Edited to add: some compounds of sodium have no effect on flame color or do not burn, so even that can have an effect on the desired results, there's really too much to consider unless you're an expert in pyrotechnics. I wouldn't try.
