Can an existing code editor DLL be used in Automation Studio, or is a new code editor required?
You can use code editor to use existing DLL or you can write your custom code in code editor.
Refer knowledge base for more details
As suggested by @Mantri - existing DLLs can be reused while creating the Automation Workflows in Automation Studio for Enterprise.
For Custom Code or Customization, you can refer Knowledge Base and also connect Edgeverve for more details.