This paper presents a method for generating a separable regression function from a nonseparable one, enabling the application of parameter estimation methods. In particular, we are interested in regressions containing parameter-dependent exponential functions -a scenario often encountered in physical systems. Our approach is based on algebraic techniques with the so-called annihilator theory and utilizes an intermediate approximation of the nonlinear part by a polynomial function of the time. Two operators are proposed to define the annihilators: time delays and differential operators. The efficiency of the proposed approach is demonstrated in a nonlinearly parameterized fuel cell estimation problem.