Yes. Once you have AMPL running, you add a solver by simply installing the AMPL-interfaced version of that solver on your system…
The procedure is equally simple if you already have a solver running, and want to later add AMPL to your system. You will need to obtain the AMPL-interfaced version of the solver, however, if your distribution did not come with it.
Generally there is no extra charge for the “link” between AMPL and solvers. In many cases the linking code is freely available.