is there any alternative OS to give more lifetime to an older iPad?
As far as I know: Not really. 
All you can do is to jailbreak it - as @Nimesh Neema suggested -, which is usually done to do some customization of iOS. It shouldn't allow you to install apps that require a newer iOS, yet you can install apps outside of the AppStore. I am pretty sure though, what you are looking for, does not exist.