If the script tool in question is also a python script you can view its source by looking at the properties context menu of the script tool in arccatalog.If that is the case, you can explicitly import the script file to your python script by saving a copy of the script in either your python path directory or just right next to (that is, in the same folder) the script you are importing it to like:import arcpy
import customPyScript
if the name of the script file you are looking for is customPyScript.pyafter you import your script you should be able to access its methods by calling it thusly:customPyScipt.method(input, output)
If you are importing a binary tool from an existing toolbox you should be able to do:arcpy.ImportToolbox(tbxPath, scriptName)
I might have misunderstood your question.What do you mean by "open [your] script in the toolbox"?