Custom coordinates can be added to the MyCoordSysDefs.fme, which is located in the install directory FME\\Reproject....i.e. C:\\Program Files (x86)\\FME\\Reproject\\MyCoordSysDefs.fme. You would add the projection information below "Examples end here". More info here: Adding a Custom Coordinate System (safe.com)

@arryl​ Thanks for your question! We now have an interactive web site that lets you define custom coordinate systems using a convenient web form, available here. It lets you start from an existing FME coordinate system or the WKT from an Esri coordinate system, and will download the coordinate system to your computer. You may have to re-open FME to use the new coordinate system.