Marie Curie IF, can secondments be hosted by my PhD institution?
As far as I understand from the official documentation, the secondment stage is meant to increase the impact of the fellowship in the host institution. From a cursory reading I don't see any reason why it couldn't take place in the PhD institution, as long as there is a solid motivation for it.

As I mentioned in another question about MSC:


  Some EU institutions have a specialized office to help applicants write good applications. If the target host institution does it's a really good idea to contact them and ask them for advice, it can drastically increase your chances.