If you have some private keys to import, my idea is to import them to a legacy wallet with a lower version Bitcoin Core Wallet, e.g. V24, then migrate the legacy wallet in V26.
You don't need to downgrade Bitcoin Core just to import a Private Key into a Legacy Wallet, because in Version 26.0 you can still create a Legacy Wallet with providing the
-deprecatedrpc=create_bdb in bitcoin.conf and set
descriptors=false when you create the legacy wallet.
You can also directly import, the Private Key from the Legacy Wallet into the Descriptor Wallet, but instead of using
dumprivkey, you should use
importdescriptors. In order to do that, You must have the form of the descriptor written properly (eg.
pkh(PrivKey)#checksum). To get the checksum, use the
getdescriptorinfo <privkey>, then import the private key as a descriptor with the
importdescriptors argument.