When you create a Knowledge Assistant type Agent, you don't to specify the type of index. The link you referred to (https://docs.databricks.com/aws/en/generative-ai/agent-bricks/knowledge-assistant) doesn't make any mention of any sort of restrictions. In my case, the files are in a volume and I have a non-zero budget specified.  There is no way to tell the Agent wizard to use the existing endpoint (to avoid quota limit issues)