How can existing contract users not be created as resources can amend a contract?

Existing contract users who weren't created as resources can amend a contract if you make them resources by using the Identify Resource feature.

You can use the Identify Resource feature to search for a user and identify the user as a resource through the Add as Resource option. While adding the user as a resource, the user can be associated with a resource organization. Once a user is mapped to a resource organization, the user gets access to the BU mapped to the resource organization.