The document shared is a generic doc which includes details on how to onboard an inbound file in audience manager.It will be also applicable for the data from another DMP but before that we need to make sure that id sync is in place for the data which we are planning to onboard into audience manager.
I understand that you are willing to upload data from another DMP into AAM which has country, segmentID, Name and description.
Please have traits created as sid="segmentID" of type onboarded and keep a signal in file as sid="segmentid" so that the respective users can be mapped correctly these traits.
If ID sync is not already done then you can upload these users and keep the datasource type as cross-device+inbound(CustomerID) so that these customers gets onboarded and when these users appear online after performing the ID sync, the offline data will get aligned to their profile and be available for targeting.
Alternatively, you can work on offline ID sync but this will require a manuall effort where you need to have a file as outlined in this documentation.
Moreover, you can make use of APIs to fire the ID sync call and make users assigned with a UUID prior. For more information on this, please follow this documentation.
Both above options are required to be completed prior to the onboarding.
I hope this answers your query, please let me know for any additional question on the given matter.