Originally Posted by
Kensagaku
Technically you can, albeit in a convoluted manner. Sparks -> Holy Swords, pop mobs in Escha - Zi'tah. Each drop (you have a 100% chance of -A- drop, as I recall) is worth 600 points when traded into the A.M.A.N. Reclaimer, meaning that every 2 items traded in you get one voucher (well, three vouchers every five items). Pick an easy mob and loop around. At 430 Sparks a pop, assuming five pops for 2150, you're getting roughly a voucher by spending 716ish Sparks, so a better exchange rate than the other way around.
Granted as stated, this is convoluted and probably not exactly what you're looking for, but just noting that there is a method to convert sparks to other currencies!