the Crystal Ball pdf is part of RooFit. If you are using RooFit you can add easly pdf’s using the RooAddition class. (see the RooFit manual).
If you are using simple RooFit you can also easily write your combined function. You can use the TF1 class and combine gaussians with exponential using the “+” operator, like
TF1 * f = new TF1("f","gaus(0)+expo(3)+expo(5)",xmin,xmax);
You can then also use the fit panel GUI to help in fitting ans set initial values of the parameters.