I've checked that you opened a ticket with our support related to the same question.
Currently, it's possible only to add Stories into Epics via the backlog of your board.
We have feature request suggesting the implementation of such ability:
...ollected feedback from users around the lack of shortcuts, and we’re here to address that: In next-gen projects, I miss the keyboard shortcuts badly. This is particularly true on the Board, but also i...
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