Add fresh yogurt to whipping cream to make creme fraiche?
No, creme fraiche needs specific cultures, which are not yogurt cultures, and lower fermentation temperature. 

If you use yogurt with Lactobacilicus Bulgaricus to innoculate your cream, and a standard yogurt process, you will get smetana (schmand). This is a dairy product with the same fat content as creme fraiche, but a different, sharper, flavor profile. It's typically used in Eastern European cuisine. 

I have never fermented cream with bifidobacteria or streptococi, but if you do, you should get yet another kind of sour cream, that's not exactly creme fraiche.

The best way to make creme fraiche is not to use other dairies, but to buy the appropriate starter culture. As long as your process is correct and your hygiene standard sufficient, you will be able to do dozens of reinnoculations without the need to buy new starter.