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.
{
    std::shared_ptr<CWallet> const wallet = GetWalletForJSONRPCRequest(request);
    CWallet* const pwallet = wallet.get();
    if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
        return NullUniValue;
    }
    if (request.fHelp || request.params.size() < 1 || request.params.size() > 4)
        throw std::runtime_error(
            "walletprocesspsbt \"psbt\" ( sign \"sighashtype\" bip32derivs )\n"
            "\nUpdate a PSBT with input information from our wallet and then sign inputs\n"
            "that we can sign for.\n"
            + HelpRequiringPassphrase(pwallet) + "\n"
            "\nArguments:\n"
            "1. \"psbt\"                      (string, required) The transaction base64 string\n"
            "2. sign                          (boolean, optional, default=true) Also sign the transaction when updating\n"
            "3. \"sighashtype\"            (string, optional, default=ALL) The signature hash type to sign with if not specified by the PSBT. Must be one of\n"
            "       \"ALL\"\n"
            "       \"NONE\"\n"
            "       \"SINGLE\"\n"
            "       \"ALL|ANYONECANPAY\"\n"
            "       \"NONE|ANYONECANPAY\"\n"
            "       \"SINGLE|ANYONECANPAY\"\n"
            "4. bip32derivs                    (boolean, optional, default=false) If true, includes the BIP 32 derivation paths for public keys if we know them\n"
            "\nResult:\n"
            "{\n"
            "  \"psbt\" : \"value\",          (string) The base64-encoded partially signed transaction\n"