Shelving filter (2nd order)
Ok, I finally found a solution after few search about sigmoïd charts and few try.
What I wanted is to modify the slope factor in the transition area without modifying the high and low border values.
So, the equations are becoming : 

coef^2 * (1 + (f / f0)^(SFactor+2) * (1/coef)^2) / (1 + (f/f0)^(SFactor+2)) for the lowshelf

(1 + (f / f0)^(SFactor+2)) * coef^2 / (1 + (f/f0)^(SFactor+2)) for the highshelf


With :

coef = multiplicator coef (gain = 10 log10 (coef))

f = frequency

f0 = cutoff frequency

SFactor = slope factor. If SFactor = 0, it acts like a classical first order shelving filter, if SFactor increases, the slope in the transition area is steeper.

Thank you for all the previous comments and the Audio EQ CookBook is also a very interesting source I'm going to study...