The only way to create a segment of subscribers is to have the information needed for filtering those stored in the list they are uploaded in.
If you want to create a segment that you have used before outside our platform you just need to include the required custom fields with information within the list you want to segment from.
If you want to import a segment of recipients that are not already present in a list in Pure360, or you don't have the custom information needed to filter them out, you can upload these contacts as a new independent contact list.