Generate XMI file with metadata API

Morning all,
I am trying to generate an XMI file automatically by the API. At moment I can read an XMI file that already exists and save it in other file.
I can't modify it :(... I don't understand the hierarchy behind the domains, logical tables, logical column.
Someone can halp me? I would know what classes I must use to modify, add or delete a table (and columns) in a schema.

Thanks to all,

Christian