I have deployed a contract in ethereum-wallet for creating my own crypto-currency. I want to ask that in future if i want to add functionalities to my currency then is it possible ?
You can deploy a modified contract, but it will be deployed to a new address. Basically, it will be another contract. As an analogy, you can think of your deployed contract like a program that you have burned to a CD(ROM). You can modify your source code and burn it to another CD, but you're not able to modify directly what has already been burned/deployed.
There are techniques for structuring you contract now, so that you can "modify" it, but your contract needs to be designed correctly for them. Some of these techniques are discussed in Upgradeable smart contracts.