Is it possible to submit a wishlist bug against a package that is not
yet into Debian ?
No.
The current package (fwknop) I am working on has not been uploaded into
Debian yet. Therefore, in my call for translations, I ask people to send
me the po files. I did not mention anything about the wishlist bugs.
You're right, this is a case where directly sending files is the only
possibility.
However, you should mention this is the call for translations.
"Please only send the translations directly to me and not as wishlist
bug reports, as the package is not yet in Debian."