Calling an add-in script from JSL can be done with the include statement. You'll need to know the addin ID. For example suppose you have an addin called MyGreatFunction, and the addin ID is com.jmp.MGF. The JSL file called by this addin is mgf_main.jsl. To see the ID for an addin click on View > Add-ins
In your JSL script you would do this:
include("$ADDIN_HOME(com.jmp.MGF)\mgf_main.jsl");