How to add items to Schematic and BOM without footprints in the PCB in Altium
The easiest way to do this is to simply change the item property 'Type' from Standard to Mechanical. This will still cause the item to output to the BOMs, but will not translate as a component with a PCB footprint.

This is something you can also set in the library definition by modifying the component's schematic symbol in its SCH library.