I'm not exactly familiar with what you are talking about. Baseband is a generic term used in data communication networks to describe any signaling method that occupies the frequency spectrum from DC to some upper limit.
So for example RS-232 at 9600 baud is an example of baseband signaling. One of the ethernet variants refered to as 10Base-T is another example of baseband signaling.
What the RF deck will do is to mix a baseband signal with the output of a local oscillator. The circuit block called a mixer takes two signals in and produces four outputs. If the two inputs have frequencies f1 and f2 then the output will contain signals at f1, and f2, and f1 + f2, and f1 - f2. Either the mixer itself or subsequent filter stages can remove unwanted components.
I think the answer to your question is, yes a baseband chip is required to create the modulationg signal that is sent to the RF chip.
The information you gave was kinda vague so this answer could be off the mark. Could you tell us what kind of system we are talking about, maybe give us some part numbers or manufacturers application notes?