If you can transform the variable to make it linear you certainly can. Or model the variable as you did [adding a non-linear term a quadratic if I understood what you did]. It is done all the time.

Ultimately the answer depends on whether the variable that has a non-linear relationship is inherently non-linear or can be made linear [transformed to be linear]. Commonly this depends on (I believe) on whether the X or the slope is non-linear.