Can I install an App that is not available in my country's iTunes appstore? I want to download and install an app on my iPhone, that is available in the US iTunes App Store - but not in my country's iTunes App Store.

Is that possible without creating a US iTunes account or jailbreaking my phone?
A common way to work around the cross-border restrictions of the iTunes store is by using pre-paid iTunes cards. These are often available on eBay and you can redeem them in the iTunes store for the country of origin (assuming you want US items, you need to buy a pre-paid card from the US.)