Sorry I misread. You want to use an existing entry at the point of Passkey creation. Good point, that is currently not possible. Nor is it possible to merge two entries (different issue) or directly migrate passkey data from one entry to another.

Maybe we could add a new button to the dialog that opens the Import Dialog instead (with a different title of course).

That is what I was thinking too, could rename that dialog to "Choose Entry for Passkey" or something similar. Technically it is post-import.

I think this merge should be done by browser extension.
I don't think there is any other way to enter passkeys into keepassxc.
However, developing issue #2202 could also help a lot, once the passkey has been created you can merge with an existing item.
At the moment it's quite boring, after enabling the passkey on github I had to copy all the attributes of the existing item into new item

@juvannx The merge cannot be done in the extension side. See the linked PR in this issue for a solution.

@varjolintu my fault, I described my suggestion in a wrong way, your PR does exactly what I wanted, I'll wait for version 2.7.8.