There are essentially three different ways to supply fresh air to your home.
The first and often preferred way is to install an ERV or HRV -- a balanced system. You mentioned your concern with an ERV freezing up, so I assume you are in a cold climate. If that is the case, an HRV may be better fit -- it's basically the same as an ERV but does not bother with the moisture exchange; just the heat exchange.
The second option is running a small duct (4" should work) from the outside to the return side of your system. Sounds like your return is right next to the furnace in the middle of the house, which I'm assuming means inside the conditioned space. It may be slightly difficult to access, but you should be able to run the 4" duct down the wall to the return plenum.
There are several different ways to do this as well. You can use a manual damper that you adjust yourself (you can close it down on cold days, etc.) or an automatic damper that will open whenever your system comes on. They even make some dampers on timers that will turn them on for x minutes every hour if the system does not run on a mild day.
The last option is installing some bath fans and putting them on timers. The downside to this approach is that you are relying on leaks in the home to supply the new air. This option should only be done after a very good air sealing job in your attic.
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