You can add as many customers as you'd like to the game.  New customers are made of 5 images that are ideally at least 256x256 pixels in size, each image should depict a happier or more "flustered" mood than the previous.  
The images should be named:
1.png
2.png
3.png
4.png
5.png

To add a new customer find "Cowtastic Cafe_Data/StreamingAssets" in the game files, and then create a new folder in that folder to place your 5 images inside of.

Copying an existing customer folder and renaming it is a good way to start.  Also, not only can you add your own customers, but you can also replace the existing ones.


--- Advanced customization ---
Customers should also have a stat.json file if you want them to be more than just default customers.  The easiest way to make this file is to copy one from an existing customer folder.

The current customizable stats included are:
"Weighted"                        - Default Value: 1000 - Decides how often a customer appears compared to others.  A value of 1 would be extremely rare, while 2000 would be twice as likely as regular customers.
"FlusteredMultipler"              - Default Value: 1.0  - Decides how easily flustered the customer can become.  Values too small may mean they cannot reach max fluster, and 0.0 means they will never fluster at all.
"RemoveEventIfCustomerIsFinished" - Default Value: true - Decides if the Customer Event stops when the customer leaves.  Many events will run forever if this is set to "false".
"CustomerEvent"                   - Default Value: 0    - Decides which event to play when the customer arrives.  A value of 0 means no event, the other 20 events are listed below.

-Customer Event IDs-
0  - None.
1  - 2x Milk Production Speed
2  - 4x Milk Production Speed
3  - 8x Milk Production Speed
4  - Regular Growth Bursts
5  - Fast Growth Bursts
6  - Big Growth Bursts
7  - Milk Drought (Extremely low milk production)
8  - Barista Mood Decrease
9  - Barist Mood Increase
10 - 1.25x Drink Sell Value
11 - 1.5x Drink Sell Value
12 - 2x Drink Sell Value
13 - Set Barista's Milk Type To "Normal"
14 - Set Barista's Milk Type To "Chocolate"
15 - Set Barista's Milk Type To "Strawberry"
16 - Set Barista's Milk Type To "Honey"
17 - Set Barista's Milk Type To "Blue"
18 - Set Barista's Milk Type To "Green"
19 - Set Barista's Milk Type To "Rainbow"
20 - Set Barista's Milk Type To "Space"
21 - Set Barista's Milk Type To "Void"

(Check this file again after updates to see if you need to update your custom customers.)