p90x and the like, are fine for general strength building. I would suggest finding a softball specific strength workout. They are out there, just have to look. If you know anyone in your area who does training, see if you can get with them to create something. Last off season, we partnered with a local gym, had them create a specific arm/core strength and speed workout, that we did 3 days a week. We worked out a price where we would come to their facility twice a week, and they would send two trainers down to the school once. It worked pretty well, and we saw some strength and speed gains.
If that isn't an option, I've had both baseball and softball players have success with Volt Athletics online training. Also, Matt Lisle recently put out his SwingFit program. Both are a little pricey, but seem to be good options if you dont have a local gym to help you out.
Hope this helps.
This what we did for DD. Her weightlifting regimen was more for core strengthening, upper back/shoulder muscles for catching and overall muscle toning than actually adding mass. The speed and agility helped with speed and the cross-fit helped with overall muscle tone. It was developed by a sports specific trainer who specialized in BB and SB...especially catchers.