I don't know much about DB2 for AS/400, or even what the current release number is, but on DB2 for Linux, Unix, and Windows you can put a UNION ALL in a view since at least version 7 (the current version is 8), and maybe earlier.
But a UNION is not supported for views, only UNION ALL. Remember that a UNION tries to remove duplicate rows, and UNION ALL does not. Many times there are no duplicates to be removed and the UNION command is used inappropriately, when a UNION ALL would be better.