Following the advice you have found what could be done is to convert the file you have encrypted with an ancient cipher to something modern:
openssl pkcs12 -in /meinpfad/meinzertifikat.p12 -nodes -legacy | openssl pkcs12 -export -out /meinpfad/neu.p12
Didn’t test it with 3.0 but might work. This will also decrypt your private key for a brief moment and pass it to another openssl command. Then you can try importing neu.p12
(This whole issue has nothing to do with Nitrokeys, it is about PKCS#12 format and breaking changes in OpenSSL 3.0)