For a while now I've been wondering if it would be possible to copy a ToolBox parts and then modify the sketches, features etc to get a "Custom" part with similar options as the Native TooBoxs parts.

What I did was copy a TB part, inside the configuration utility into a new folder. I then Renamed the model that was copied. I then went in and deleted the existing sketches and made my own. 

When I first placed the part from the toolbox it gave me "Blah, blah, blah not found". I just continued to add named features, sketches, sketch variable names etc etc until I could place the new part with no errors.

Low and behold, it works.

It's not truly a custom part because you are stuck with all of the existing names in the database. For instance I copied a metric SHCS. My "Variables" that show up when I place the part are named what they were in the SHCS and are asked for by that in the placement dialog. See below. All the circled stuff is hard coded and can;t be changed. However all the options for these are editable in the Config Toolbox database.

By changing these variables, sketches and features you would have a pretty large range of things you could do. Below is a quick example of what I did with a single feature. You have complete access to equations as well as several features you can work with, rename or make different features. The only thing I changed below is I had two different sizes with a few different Lengths and Thread lengths. The length variable controlled the length of small diameter and the Thread length variable controlled the diameter of the large disk.

Inside the database you have 10-15 unique variables that are associated with the size but can not be changed when placing the part. I was planning on creating an Air Flow control fitting. The Size would probably be based on the Hard fitting, 1/8NPT, G1/8 etc. You could then use the Length variable for Size of tubing and Thread length for Meter in or meter out. 

By allowing custom configurations and custom designations you could import the names for each config you want and that would show up while placing as well and place the proper config name in the model.

I've only messed with this a tiny bit but so far I think it may be somewhat useful, although I suspect a bit buggy.