In declarative programming languages based on the constraint programming paradigm, computations can be viewed as deductions enhanced with the use of constraint solvers. However, admissible constraints are restricted to formulae handled by solvers and thus, declarativity may be jeopardized. We propose a domain-independent scheme to extend constraint solvers so that they can handle alien constraints, i.e., constraint involving new function symbols. This mechanism, called Solex, consists of a set of symbolic rule-based transformations: they add and deduce syntactical as well as semantic information related to alien constraints, complete the computation domain, and purify constraints in order to allow solvers to cope with alien constraints. These transformations can be seen as elementary solvers, and thus, Solex is a collaboration of these several solvers with the initial solver. Some extensions of computation domains have already been studied to demonstrate the broad scope of Solex potential applications.