How to set box api id as an owner for an existing folder?

 yes, you could use a JWT application to create an app user (or use the service account) and then make that user the owner of the folder.

1. Create a JWT app https://developer.box.com/guides/applications/custom-apps/jwt-setup/
2. Using JWT,  get the current user's ID https://developer.box.com/reference/get-users-me/ 
3. Next, use the As-User header to act act the original owner of the folder https://developer.box.com/guides/authentication/jwt/as-user/
4. Then, as that user create a collaboration, inviting the service account of the JWT app using it's user ID https://developer.box.com/reference/post-collaborations/
5. Then, still as the same user, update that collaboration to give the service account the "owner" role https://developer.box.com/reference/put-collaborations-id/#param-role

Now, all users still have access to the folder, but a new user has been added that belongs to a JWT application, and that user is now the owner. 

You can learn more about users, app users, and service accounts here.

https://developer.box.com/guides/authentication/user-types/app-users/

Hi , The app is Enterprise Integration app, and set the OAuth 2.0 with JWT (Server Authentication), and I'm using the Python SDK. From the link, the curl it ask ACCESS_TOKEN , and now I can only use the json file for the authorization, not sure where to get the access token, can not goto the next step, could you please give some advice? Just want to transfer the folder owner to the box service account id, thanks.

 the SDK will handle converting the JWT config into an access token for you. Most of those pages have examples in Python.