In Projects>Actions>Import XML. Once past choosing the file, you will hit next and come to the screen below. On resources there is a drop down. You can select from Insert New, Updating Existing, Keep Existing, Do Not Import
Project Data Import Actions
You can configure the action that is taken for each project data type imported with a project. The selected actions apply to all projects being imported from the P6 XML file.
When configuring the project data import settings, you can choose from the following import actions:
Update Existing: If a matching object is found in the project, it will be updated based on data from the import file. If no matching object is found, the object will be created as new.
Keep Existing: If a matching object is found in the project, it will be kept and its data will not be updated. If no matching object is found, the object will be created as new.
Do Not Import: The data type is ignored and is not imported. However, if a matching object is found, any associated data (such as resource assignments) will be imported.