Can't add a custom lookup field to SSP "Log A Case" page layout

I'm trying to add a custom lookup field onto the Self-Service Portal's "Log a Case" page, but the page layout editor doesn't give me this field as available for adding (it's just missing entirely from the drag-and-drop banner).

Is this because of the field type being lookup field? I noticed there were no other lookup fields available for adding in the banner neither.

Thanks for any info on if this is possible to do or not!