FI add-ons
 

To be able to instruct on existing ratings I hold like night , aerobatics etc when I hold a EASA FI (a) do I need to have a lesson with a FI-FI to allow me to instruct on ratings I already hold? Like for example to instruct towards a night rating

You need an FIC for both. At least you did until 31/12/2020.......................... The Night add on is quite short but I believe the Aero is at least 5 hours.

Could this be done at any (easa) flying club or does it have to be done at an ATO?

It needs to be done at an ATO with approval to conduct FI/CRI courses.

I see from your location you're in Norway, but for UK licence holders Standards Document 31 [here] is the place to look for who can and can't help.

In order to add the FI qualification you have to demonstrate to a FIC Instructor that you can teach aerobatics. It is not an approved course and doesn't need an ATO. FCL.905.FI FI(g). For the Night Rating you need to have completed the 1 hour night instruction which may have been completed on the FI Course, more commonly it is an additional hour at night at an ATO with the FIC approval.

Didn’t do the night rating instruction as part of the fi course.

So practically I can do those two on two separate lessons with any FI - FI at our DTO?
FI-FI being a FIC instructor , although we don’t offer flight instructor courses being a DTO

The night is part of an approved course so would have to be conducted at an ATO but the Aeros could be conducted by an FICI infependantly as it only requires a sign off.