The ideas is for koin to keep its own value level, independent even of the dollar.  But at first it could be tied or automatically convertible to the US dollar.  Somehow it must be easy to buy items according to own's own national currency.  

A portion of ones koin-money could be assigned into dollars, thus tied and guaranteed to act as dollars—or any national currency.  But to reverse the process would mean re-buying koin at present rates.  This is simply to make buying items in one's own currency or country much easier.   

This might be best handled by those third-parties who provide commodity backing.  They might also buy up money in various nations and pay in behalf of koin users.  

The network could guarantee simple conversion to almost every currency.  At a minimum this would require some third-party or price-feed.  Now, lets be clear here.  The purpose of koin is to keep things simple, not to do everything.

Assigning koin-money into dollars may simply mean cutting up some koin into dollar amounts.  To keep the value neither up or down could be done by putting in or taking out tiny koin fragments, perhaps by an agreement with someone predicting koin's advancement in value.  But a exchange is still needed to actually spend the koin, however configured, if a merchant requires dollars/euro/yuan/yen.  (A guaranteed direct transfer of equal values could make for easier exchange.)

/The agreement is basically a electronic contract requiring them to keep a 'bag' of koin money, equal to a US dollar, etc.  Either they take out or put in koin to keep it even, up to 10 times the beginning value.  At this point, where koin is 1/10 the relative value of the dollar, this 'bag' or koin-dollar has failed to keep full value. (Or koin-euro, koin-yen, koin-yuan, koin-pound etc) 

/Or this is my thoughts.  Of course one should buy back (convert back into) koin before letting it fail, but if the dollar beats it by 10 times, chances are it is a really really old koin-dollar, or the koin network has failed.

Koin already plans on having outside exchanges spring up.  The internal purchase system, using only bitcoin/crypto, simply serves as a means of assessing and managing value.  Certified third-parties may be useful, however, for handling other currencies or providing price-feeds.  (They are already used for investing liquidity funds, providing commodity-backing and identity verification.)

It is hoped that merchants will simply eventually agree to take stable koin, with no exchange necessary.