To get the Project Wizard to accept a user-defined datum, you need to store it with an empty coverage and then reference it with the wizard option 'Project my data to match existing data'.
The datums list is hard-coded.
Solution or Workaround
- In ArcInfo Workstation, run the CREATE command to create an empty coverage.
- Run PROJECTDEFINE on the empty coverage and set the DATUM to USER_DEFINED with the appropriate values.
- In ArcToolBox, start the Project Wizard, select 'Project my data to match existing data', and click Next.
- In the Dataset field, specify the name of the empty coverage containing the user-defined datum.
If later projecting needs a different user-defined datum, modify the prj.adf file located under the empty coverage's folder. You can use a text editor for this.