Can I use my full frame digital camera to set up the exposure for my medium format film camera?

Yes, you can use a digital camera to take test shots in much the same way pros used Polaroid backs for many years. Just keep in mind that both film and digital have some leeway with regard to ISO. Many digital cameras can be 1/2 stop or more less sensitive than they claim for a particular ISO. One of the things DxO Mark tests is actual sensitivity for each full ISO stop. Here's a link for the Nikon D610 and Canon 6D (You'll need to click Measurements-->ISO Sensitivity to see the comparison).

Different films also vary slightly from the exact ISO they are rated at. You might run into a case where both the film and digital vary by the same amount in the same direction and so they are both almost equally sensitive or you might find a combination where the film is more sensitive and the digital camera is less sensitive and wind up with differences that add to one another.

Do I have to convert anything, because of the different medium (6x4.5 120 film vs 35mm digital sensor)?

No. Nothing must be converted due to the different format sizes.

However, if contemplating exposures for film longer than about 1 second the Schwarzschild effect, sometimes referred to as reciprocity failure, must be taken into account. This can very significantly impact exposure times, and it varies by the specific film in question. The manufacturer of your film should be able to provide information regarding how much compensation is needed for longer exposures.