Adding an external key pair to a local bitcoinj wallet
After searching for a while I found this type that takes a private key (in base58), required network parameters, and constructs the public address. Please refer: DumpedPrivateKey. This was enough for me at least. I construct an address from the key so formed using the toAddress method exported from the type key.

DumpedPrivateKey vanityKey = DumpedPrivateKey.fromBase58(networkParameters, "5JXFsJEAiqtTv95rSmw3KSJeAZ97SsmKWTQq5yqBzioSJ1guUHK");
ECKey ECVanityKey = vanityKey.getKey();
Address vanityAddress = ECVanityKey.toAddress(networkParameters);