I just made a pattern that I want to add to an existing custom color library and I wanted to know what is the best/quickest way to do this. Thanks
Color Groups or those folders within the Swatch Panel can not contain patterns or gradients. If you are trying to add a pattern to a color group, that's not going to happen.
If you are trying to add a pattern swatch to an existing .ase swatch file, you need to add all the existing swatches to the Swatch Panel, then add your new pattern and then resave the library. You essentially have to make the Swatch Panel mimic how you want the library to look, then save it. There is no easy way to just add a new swatch to an .ase file.
If you are trying to add a swatch to a library in .ai format.... open the actual .ai file. Add the swatch to the Swatch Panel and then just save.
If you are trying to add a swatch to one of the CC dynamic libraries... you can't add pattern swatches. The easiest way to add a pattern is to create a rectangle, fill it with the pattern, then drag the rectangle to the Libraries panel. It will load as "artwork" but it will contain the pattern. You can then just drag the rectangle from the Libraries pattern in the future and the pattern will auto-load into the Swatch Panel.
---- the above is why I generally save my libraries as .ai files rather than .ase -- the ease of how additional swatches can be added.