Shouldn't it be possible to add a custom function in Sql Server so that I could still use Empty([feildname]) in an expression?

Select 
 Case When Empty(Activejobitems.part_no) Then
   Activejobitems.description
 Else  Activeparts.description End as description
From JobItems
Activejobitems  Inner Join Jobs Activejobs On Activejobitems.job_num = Activejobs.job_num
Left Outer Join parts Activeparts On Activejobitems.part_id = Activeparts.id  Inner
Join schedule On Activejobitems.job_num = ScheduleRecords.job_num
Inner Join machines On ScheduleRecords.mach_num = Machines.mach_num
Where ScheduleRecords.mach_num = '   ' AND
  ScheduleRecords.status <>  'C'  AND
  Activejobs.status =  'A'
Order By [ScheduleRecords].[mach_num] , [ScheduleRecords].[order_mach]