Creating a user code can be complex sometimes. Some rules, such as the length of the user code, will cause some trouble when you have many users.
The length of user code in SAP Business One used to be 8 characters, but after SAP Business One 9.2, the length extends to 25!
Go to Administration > Setup > General > Users and you are allowed to create user codes of up to 25 characters.
If you are interested in a situation in which a business partner is your customer as well as your vender, please visit Connected Vender in SAP Business One.