I have met one of the lead developers of Desperados 1 but 10 years ago, he didn't agree to give us the editors because he was not allowed to. Now, he's not able to anymore, or maybe he says so. Anyway, he told me additional missions were created but never released.
One solution would be to build a petition for Desperados' source code to be open sourced so everybody can get into it and create new missions. It would be cool because there are many movies and so on for inspiration. And this great game needs more.