Ok, first download Soaprman's FEFTwiddler software. It's here:
https://github.com/Soaprman/FEFTwiddler/releases
Then visit this page:
https://gbatemp.net/threads/fe-fates-fe14unit-recopilation-thread.480336/
You will be able to find the amiibo characters you will need or any other character.
Handling :
1 / extract your backup using JKSM
2 / Remove the card sd
3 / On the computer, make a copy of your backup so that it is not corrupted if there was any error.
4 / Go to the software then open your "chapter" file that you want to edit.
5 / Go to tab number 2 then add the desired character
6 / save your new file on the sd card at the same place where there are your backup of fire emblem fates
7 / put the card sd back into the console and then import the backup with JKSM
8 / Have fun
and if there was an error, replace your backup file with that saved on the computer and redo step 7.
NB: I am in no way responsible for the loss of a backup or any error.