You could say, "A warm welcome to Professor [name]."
I don't think there is anything wrong with it, but it's not what I would say.
To me, this version is more like an announcement -- we giving Professor [name] a warm welcome. However, the professor will understand that you are addressing the welcome to <him> or her.
The version, "Welcome, Professor [name]." addresses the professor directly, and sounds warmer. You could say <it> when someone comes to your house as well as in formal occasions.
Nonetheless, you could use either one, according to your preference.
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