iVPN Connection Problems
Doing so would require being able to access your iMac from remote places, thus configuring either a DMZ host, or port forwarding on your router. Either way it will be a headache. 

Also if you're on a residential connection note that the public IP address can and will change. I recommend a dynamic DNS server for that.  

Assuming you can get that all setup it's as simple as setting up VPN on your iDevice.