Is there a way to utilize an Airport Extreme in order to VPN in? Not using MobileMe
The Airport Extreme is just a router - it doesn't provide any support as a VPN endpoint itself.
You can use a computer with the server side of the VPN connection (eg. OpenVPN, VPN Access in Mac OS X Server, etc.) to truly VPN in. You will need to tell the Airport Extreme to forward the necessary ports to that computer.
Of note Mobile Me's Back to My Mac service connects to directly to your Mac and has nothing to do with an Airport Extreme, the reason Apple recommends it is that they "know" that an Airport Extreme plays nice with UPnP and handling IPv6 traffic.
Short Answer: No.
Longer Answer: AirPort Extreme and Time Capsule products do not have VPN access concentrators. There are two workarounds: 1) port forward to a separate VPN access concentrator you deploy at a static IPv4 address behind the NAT; and/or 2) sign up with a free IPv6 tunnel service and use IPv6 to reach your local network. (You can make your local IPv6 network "private" by turning on the the IPv6 simple security feature. It's called "Block incoming IPv6 connections" in the AirPort Utility. I don't recommend using it unless you know what it does and doesn't do for you.)