To add a custom projection from an PRJ file:

Load the PRJ into a Geographic Coordinate System with the following keyin:

MapCSUtilities ReadWKT <filespec> 

For example:  MapCSUtilities ReadWKT C:\Temp\2462.prj

This keyin will replace an existing GCS without prompting you to reproject or to change the storage units. Once it will become the active GCS, take a screen shot of the parameters, and fill this into your custom GCS.

Note that the datum must exist in GCS library. The WKT Command will first look for an equivalent datum and if found it will be displayed saying that the command succeeded, else it will show failed command message.

If you want to create new datum , please refer to:  communities.bentley.com/.../9002.add-datum-andor-ellipsoid

Note : The keyin does not accept spaces, special characters in the folder name.