The Short Answer
Nope, not possible, or at least compicated & tedious, because Android defaults to WiFi since it's generally faster, more reliable and has no data cap
You may want to look into this app. While it requeires a rooted device, it allows you to leverage on both your WiFi & 3G to download at the same time.
The Long Answer
As t0mm13b has suggested (and I eagerly defer to him), if you're really serious about this and have the necessary expertise, you can
modify the source code to remove the mutually exclusive flip/flop and also at the RIL layer
Unfortunately, I'm not qualified enough to do more than point you in the direction of the Android Developer Page if you decide to go down this route.