If you use custom libraries for brushes or other assets in your Illustrator workspace (symbols, swatches, graphic styles, etc), it’s easy to make sure they appear right off the drop down menu as long as you know where to save them. By default, this is not where they are saved and you have to manually search for them every time you want to load them, but it’s easy to force them into a different spot for quick access all the time. Once you have created, purchased or inherited your custom libraries (these libraries will be saved with the standard .AI extension), you will want to save them in the following location accordingly:

Mac OSx

Macintosh HD > Applications > Adobe Illustrator (your version here, i.e. CC 2014) > Presets > en_US (or your language) > Brushes (or other folder for the appropriate type of library)

PC

C: > Program Files > Adobe > Adobe Illustrator (your version here, i.e. CC 2014) > Presets > en_US (or your language) > Brushes (or other folder for the appropriate type of library) By putting files in the Brushes folder, they will automatically be available from the drop down menu from the panel.  You can even take it one step further and save the libraries named strategically so they land at the top of the list (files are listed alphabetically, so number 1 & 2 will appear first as shown in this example):