Well, I haven't tried it myself, but since no one else has replied I'll offer up this...

I've been using Craig Biggio all year as my starting third-baseman. He leads my team in errors, but apart from that, he's been reasonably effective defensively. The manual has ratings for moving a guy from second to third, as you already know. Supposedly, making a switch that isn't listed results in a pretty serious degradation of defensive abilities. But, there are plenty of guys listed as second baseman with brutal defensive ratings, so if Coskie has good numbers at third, his ratings at second might be passable. If you're willing to take a hit defensively, I say give it a try for a homestand.