You need to use in-line view or subquery factoring. For example:
with t as (
functionconvert(e.employee_id) as employee_number
from employee e
when employee_number like "ad%" then "emply from ad"
when employee_number like "ab%" then "emply from ab"
If you define a column alias (such as employee_number) in a query or sub-query, then you can use that column alias in the ORDER BY clause. That's the only place you can use the alias in that same query or sub-query. If you want to use it anywhere else (as you want to use it in the CASE expression), then define the alias in a sub-query, as Solomon showed. Then you can use the alias wherever you want to in a super-query.