Parameter restrictions

Suppose I have a model with parameters a,b,c. I want to estimate a and fix b and c. But (for example, so that the steady state satisfies certain properties) I need to constrain b to be a function of a and c, possibly an implicit function like F(a,b,c)=0. I suppose I could pretend b is an endogenous variable and add F(a,b,c)=0 to the model system, but is there a more efficient way? Thanks.