How to add reference of other units from one project suite into another suite?

Add usenet statment on first line of script unit. The following allows access to code in Purchasing.sj. Please mark as solution if this solves your problem.

//USEUNIT Purchasing

Here's the starting point within the documentation for the script extensions. http://support.smartbear.com/viewarticle/69987/

You've already done the hard part: writing your code.  All you need to do is write up the XML file, and package it all up.  You'll need to rename your unit files to .js from .sj.  Please, please, please remember that you cannot call other script extensions from with your script extension, and you cannot call back to your project from the script extension. 

Here's the link for the description.xml structure: http://support.smartbear.com/viewarticle/68445/

When you've got the files ready, just zip them up and put the zip package in the ScriptExtensions folder with a .tcx extension.