IF 2016: I will be applying for a standard European Fellowship, for which I understood there is no requirement for a letter of invitation/support from the PI of XXXX I will propose for my secondment. However, and even if this is not required, would it be possible for me to attach such letter of support at any point during the application?

The letter is indeed not required for EF-ST. It is theoretically possible to attach it to the Document 2 of part B.
(Answer provided by REA)