Add conditional column to new table (generated by function UNION)

Hello! I need to add a conditional colunm to a table generated by the UNION of 24 other tables.

Is there a way to add this column direct to the consolidated table (generated by UNION function) or maybe a way to add this colunm to all 24 tables simultaneously?

There will be so many conditions in this new column and it would be really hard to add it one by one on the original 24 tables...

Thanks!