It would be difficult to do it precisely with doing some sort of frame-by-frame touchup.
But you could use a matte (like the garbage matte or the track matte). Just place a clip on your timeline and then another copy of it on the track directly above, apply the track matte around the object you don't wan't to blur on the clip on Video 2, then apply the Gaussian Blur to the clip on Video 2.
If you understand how to use mattes, that will make sense. Otherwise, it might be difficult to explain on this forum.
My step-by-step special effects book goes into creating matte effects, if you're interested.
In PrE, I think that Steve's suggestion is about the only choice that one has. There are some DOF (Depth of Field) plug-ins, but most are available for Adobe After Effects, or for Still Images, Adobe Photoshop. As there are not that many suppliers, who write plug-ins to be used with PrE, as the host application, if such exists, it might take some searching. I just checked on NewBlue FX, a supplier of PrE plug-ins, but did not see anything like DOF - however I might have missed that, since they DO have a lot of various plug-ins.
Though not exactly what you are looking to do, this article discusses a general use of an altered Duplicate and Track Matte Keying. In this case, rather than making it look like the DOF was adjusted, the Track Matte is just a "circle" to separate a particular subject (say a football player) from the background. For your use, the Track Matte would need to be more intricate. Still, the general technique is covered: http://forums.adobe.com/message/4575874#4575874
Steve's article in his Tips & Tricks book goes into even more detail, and for that one "Trick," would be worth the $.
Steve Grisetti wrote:
"... apply the track matte around the object you don't wan't to blur on the clip on Video 2, then apply the Gaussian Blur to the clip on Video 2."
Are you shure that Gaussian Blur has also to be on video 2 (and not 1) ?.
The solution works for me but only if vid2 has the (garbage) matte and vid1 the blur ... ?
Because the border of sharp and blur is very thin (not gradually ...) I'm not gone a use it (or is it possible to change this also for a more natural effect ?
You can also put a garbage matte on a white color matte.
then you put the Gaussian Blur on the Matte in order to feather out the edges.
Easiest and more neat is to make a matte in the Titler.
Track 1; original clip, track 2 duplicate clip and track matte effect set to track 3. track 3; the matte with the Gaussian Blur/Fast blur.
I did not see DOF work with the Track Matte Keying, in Cool Tricks & Hot Tips, but the vast majority of uses of Track Matte Keying DO have the Effect(s) applied to Video Track 2's Clip, and then to hold parts of that Effect out of the Clip on Video Track 1. DOF is an exception.
When I read the comment, I did not pick up on that juxtaposition of Track material, because most often, one IS applying the Effect to the Clip on Track 2, not Track 1.
I think that Steve handled it pretty well, and we all get to smile...