It's currently not possible to import (solvable) SegWit addresses (P2SH and native) using `importmulti`. I believe that's a shortcoming we should address before release.

My suggestion would be for P2WPKH (P2SH and native) to be supported without any extra arguments. This means that a P2WPKH address/script with corresponding private or public key should work. For P2SH it means checking if the supplied P2SH address perhaps matches the supplied public key or private key when converted to P2WPKH first.

To support P2WSH, I think we need an extra field "witnessScript". When supplied, it would either be required to match a P2WSH address/script directly, or if the address is P2SH, indirectly.
"If 'label' is specified, assign address to that label.\n"
            "\nArguments:\n"
            "1. nrequired                      (numeric, required) The number of required signatures out of the n keys or addresses.\n"
            "2. \"keys\"                         (string, required) A json array of bitcoin addresses or hex-encoded public keys\n"
            "     [\n"
            "       \"address\"                  (string) bitcoin address or hex-encoded public key\n"
            "       ...,\n"
            "     ]\n"
            "3. \"label\"                        (string, optional) A label to assign the addresses to.\n"
            "4. \"address_type\"                 (string, optional) The address type to use. Options are \"legacy\", \"p2sh-segwit\", and \"bech32\". Default is set by -addresstype.\n"
            "\nResult:\n"
            "{\n"
            "  \"address\":\"multisigaddress\",    (string) The value of the new multisig address.\n"
            "  \"redeemScript\":\"script\"         (string) The string value of the hex-encoded redemption script.\n"
            "}\n"
            "\nExamples:\n"
            "\nAdd a multisig address from 2 addresses\n"
            + HelpExampleCli("addmultisigaddress", "2 \"[\\\"16sSauSf5pF2UkUwvKGq4qjNRzBZYqgEL5\\\",\\\"171sgjn4YtPu27adkKGrdDwzRTxnRkBfKV\\\"]\"") +
            "\nAs a JSON-RPC call\n"
            + HelpExampleRpc("addmultisigaddress", "2, \"[\\\"16sSauSf5pF2UkUwvKGq4qjNRzBZYqgEL5\\\",\\\"171sgjn4YtPu27adkKGrdDwzRTxnRkBfKV\\\"]\"")
        ;
        throw std::runtime_error(msg);
    }
    LOCK2(cs_main, pwallet->cs_wallet);
    std::string label;
    if (!request.params[2].isNull())
        label = LabelFromValue(request.params[2]);
    int required = request.params[0].get_int();
    
    const UniValue& keys_or_addrs = request.params[1].get_array();