This feature is not currently supported through a 'user-friendly' method; the present API allows easy retrieval of existing footnotes but not foot/endnote creation.
Of course it's possible to create a note through the low level OODoc::XPath methods (and a bit of knowledge of the OpenDocument Format), but it's not a smart way.
If such a feature is regarded as useful for a future release, please create a ticket in the issue tracker
(http://rt.cpan.org/Public/Dist/Display.html?Name=OpenOffice-OODoc),
describe the desired feature and select the 'wishlist' option in order to avoid confusion with a bug report.

As said here, offset specifies the numeric position of the note in the text of the target element.

The offset option is the index of the character which the note should be placed before.
If offset=0 (the default), the note is placed before the text.
If offset=-1, the note is placed before the last character.
If the insertion point depends on the content, the application may compute the offset using index() or a regex-based search.

The meaning of the offset parameter in setNote is consistent with setBookmark.