PF: Is it possible for a MSCA Postdoctoral fellow to be included as an associate in other project within their host organisation, without being paid or having an additional contract?

This is not possible considering Article 6.2 of the Grant Agreement – A1 - (b) the recruited researchers comply with the following conditions: (iv) be working exclusively for the action, unless part-time for professional reasons has been approved) (p. 22).