Doubt in mixer measurements

In mixer on-wafer measurements,

- There is a test where a RF generator is connected to the RF input port,
so to add a DC component to the RF signal (so the signal would be
suitable to be applied at the input of a MOS) I can use a Bias Tee.

- There is another test where a VNA is connected to the RF port. In this
case, is it necessary to add a DC component ? If yes, how I can add
the DC value needed to polarize the input MOS of this input RF port ?

you can use the bias features of your VNA.
Usually VNA have , on the back panel, 2 connector used to bias the dut.