User defined libraries are plain AI files. You can just open them in Illustrator, add to the swatches (or brushes or symbols) window whatever you want and then save.
Thank you for responding, however I am not exactly clear on what you mean by "libraries are plain AI files". For example, I cannot find a file with the same name as my user defined library name that I have created. I can find the preinstalled swatches listed under Adobe Application Support... Library... Swatches, but a user defined folder is not present and the library sets that I have already created are not visible. I have tried searcing my Mac for a (name).ase file and still no luck (at least for the name I was looking for). I can open the user defined library from the Swatch library icon on the bottom left of the Swatch panel, but when I open this I cannot drag and drop add new swatches directly to this library. As stated above, I have to add each pattern, one at at time to the normal swatch panel, then add back the previous ones I have saved from the user defined library to the swatch panel and then save threm all as an updated user defined library. Thus by repeating this process, it builds up the user defined library one at a time.
I am using Illustrator CC if this makes a difference.
This is OS 10.7 or higher? The user library is hidden by default in those versions. You first need to show it then look for your libraries.
Do a search on Google how to unhide the user library.
I can see how to work with this.
I open the library file, drag the other open png files onto the library file worksheet and then drag the images into the swatch window.
Not exactly intuitive at first, especially since files are hidden, but it works.