Determine a Value from a customized field in a select statement.

Try several possibilities:

select c.*, t.[Days Open], case when [DAYS OPEN] > 60 then 1 else 0 end as [60DAYSORMORE]
from d_case as c
cross apply (values (case when c.[Case End Date] is null then DATEDIFF(day, c.[Case Start Date], getdate()-1) else DATEDIFF(day, c.[Case Start Date], c.[Case End Date]) end)) as t([Days Open])

---------

select d.*, case when d.[DAYS OPEN] > 60 then 1 else 0 end as [60DAYSORMORE]
from 
(
	select c.*,
	case when c.[Case End Date] is null then DATEDIFF(day, c.[Case Start Date], getdate()-1) 
	   else DATEDIFF(day, c.[Case Start Date], c.[Case End Date]) end as [Days Open]
	from d_case as c
) as d

---------

; with Q as
(
	select *,
	case when [Case End Date] is null then DATEDIFF(day, [Case Start Date], getdate()-1) 
	   else DATEDIFF(day, [Case Start Date], [Case End Date]) end as [Days Open]
	from d_case
)
select *, case when [DAYS OPEN] > 60 then 1 else 0 end as [60DAYSORMORE]
from Q