If you just need to fit the parameters, you can use empty functions such as:
y = f0()/(f1() + exp(f2()*x)) * ...
Another option would be to provide some of the forms you want as inputs, such as:
y = f0( x, exp(f1()*x), exp(f2()*x), ... )
Finally, you could try entering in a desired expression in the prior solution box at the bottom of the "Set Target" tab. Just enter with random numeric values or guesses for the parameters a,b,c, ...
Michael