Yes, there is a way to convert them into other type of coins! Just hook up a second controller, and wagger for coins against a second player. You can read how to wagger for coins in your game manual.
However, if you have already beat the game, and bought all 676 koffins in the Krypt, and have completed konquest mode, then you might have a bunch of coins with nothing to spend them on, or to do with. I have had as many as 20,000 coins each....
Here is a good way to get rid of them. Just buy some old koffins in the krept that you have already purchased. Koffin "UR" is 6,500 blue koins, buy that one a few times to get rid of some of your blue koins. Koffin "SA" will get rid of some red koins. Koffin "XG" will get rid of some green ones. These are koffins that cost thousands of koins. You get nothing new that you don't already have, but it does get rid of koins......Talk about koins to burn baby! I just wished I had all those koins at the beginning of the game huh?
Another way to help someone in need however is to take all those thousands of koins that you have, and play against someone who is down on his luck. Let him wagger for some of your coins, then if he beats you, he can take all those coins away from you, and use them in his game. Help a poor fellow in need! This is like making a donation to the mortal kombat families of america. So with that, I depart........

damn :( None of my friends play MK:DA, so im stuck. I was hoping you could somehow convert the koins from say gold to platinum.. as that seems my only hope :|

Ive got over 5K of jade, 3K of saphire and 4K of ruby that are just not being used. I have 58 koffins left, with 55 of those being platinum and onyx .. is there a way to obtain these koins without having to win them from friends, or the mini games?

Word on that. I don't even have a 2nd controller/mem card so yeah. I got 10k worth of gold I don't need, yet all I need are 2k worth of Onyx & Plat's and I've unlocked the Krypt totally.