Gary,
I assume you are considering adding the steel beam to support the trusses at midspan.  This is not a good idea as it is likely the trusses were not designed for a point of bearing in the middle (some truss styles can take it, some can't).

For such a modification, I would suggest that you get a local structural engineer to look at it.  He can guide you in the modifications and have them meet local codes and standards.

It is relatively easy to modify a truss in such a manner that forces and stresses are redistributed in a dangerous manner. I'm sure you don't want that!

Ron

I agree with Ron.  At first glance an additional support might look like a good idea, but if you start putting members in compression that were designed for tension, well................. that's a very bad idea.   

I second both opinions. I had the opportunity to strengthen a O.w.s.j. When I analysed using a temporary support in middle. Every thing gets screwed up. I vaguely remember that support at top chord only was reasonable. Than I added angles on web and rebars on T & B chords. Gettonna bottom line is "It is better to structurally analyse the real situation"