Need help with complex column creation command

Hello, all

I need help with a complex column creation command and SQL anywhere help is not sufficient for it.

Here is the situation:

I need to write a generic DDL "alter table" command, which can add/modify columns without knowing in advance if they already exist in the destination table.

Is there a command, which looks like:

alter table "table1" add (on existing modify) column1 <datatype> <default> ?

Thank you,

Arcady