Can you create new instruments in a category?

I'm just setting up Synfire and I get how you create a sound and then connect it to an instrument in a category.

However, in the default category you get when you create a new sound bank, I'm missing many of the intruments I need.

For example under brass there's only tuba, bones and trumpets, missing cimbasso, bass trombone etc.. same for woodwinds, strings only have violins, no 1st second, solo, 1st chair, divisi sections for all string instruments etc..

How can I add new instruments to the category so I can assign everything, or is there another way to do this?