A) Is it possible to say that a wave pulse (waves packet) has one particular wavelength? B) How are wave packets built (mathematically speaking) ?

Answer: A) we can write a wave packet like Y = ∑aₙ*cos (w*n*t)

So you have lots of different wavelengths here, and you think that a wave packet has a unified frequency, but not wavelength.

B) mathematically speaking you will write a wave packet like i did up there, has a sum of many waves, where for each one you can have an intensity aₙ and a different frequency, the only thing you must see is that all the waves you are suming are moving in the same direction.