You can contact a feed provider either by e-mail (if they allow it), private message or by clicking on the "report a problem" button on their feed page. Choose "other" for the problem type and then fill out the text box with your frequency request for them to consider. As feed providers are volunteers, they may or may not respond right away or even honor the request to add a frequency, but it's worth asking. As a provider of feeds here, I try to respond to similar requests myself as quickly as I can.
