> Does the Firebird SQL support adding new columns to a table after
> there is already data in the table?

Yes, you can use next syntax:

alter TableName add ColumnName ColumnType;

e.g.
alter Document add PayDate Date;

For some reason, you must do it in single mode, no other users must
be connected. I recommend you initialize your column in some default
value

update Document Set PayDate = '01.01.1900';


Agibov Dmitry