Thats not exactly tricky to do on the server, theres a few options really, keep it as ushort so that you dont have a problem sending it to the client, but whenever you are making purchases you do pkp * 10000 and work of that figure, which means that max would be 655kk, not exactly a huge number, but realistic.
Other option would be to use uint's and then pkp / 10000 in order to please the client.
Thats how i would do it anyway, although i wont because i dont really like the idea of pkps as a currency.