Just use CROSS APPLY to introduce the expression, and then you can just give it a name you can reference.

Here's a quick example illustrating it:

SELECT object_id,

case_result,

--can be referenced by name now

derived_from_case='Simple concat: '+case_result

--can also be referenced by name in other operations

FROM sys.objects O

CROSS APPLY

(SELECT CASE

WHEN O.object_id<100 THEN 'Less than 100'

WHEN O.object_id>=100 THEN 'Greater than or equal to 100'

END

)cross_applied(case_result)

In 2008 on, you could also CROSS APPLY VALUES instead of doing the SELECT.

Cheers!