Adding A Screen Field

How do,

I am trying to add a new field to a new employee infotype screen. This field is for display

only (it will actually fetch data from PA0008).

Can a display only field be added? i.e. it doesn't have to be linked to the base table.

I add a new field on the screen called 'TRFST', and when I reference this field in the PBO area

it falls over saying that this field does not exist.

How can a display field be added?

Any ideas?

Thanks

Andy