There's no point in trying to woo an assistant, or even the host. They don't typically decide who will appear. Opon reaching the proper party, you'll only have a minute or two for your pitch. Be polite, persistent, and prepared. Offer to help set up a program.
Many shows depend on a panel of complementary or opposing guests.
As the expert, you know the field better than the producer does. By volunteering to take over responsibility for setting up a program, you put yourself in a power position. An extraordinary program can often be developed around dissimilar viewpoints, or with related experts. Prepare for the hard questions. This way you'll never be caught off guard.