If you are running bitcoin core you can import it there. If you are importing many addresses you can import them with the following rpc call:
importprivkey [label] [rescan=true]
Adds a private key (as returned by dumpprivkey) to your wallet. This may take a while, as a rescan is done, looking for existing transactions. Optional [rescan] parameter added in 0.8.0. Note: There's no need to import public key, as in ECDSA (unlike RSA) this can be computed from private key
What you could do to speed this up is set rescan=false so that it doesn't rescan for every address.
I think you can then just restart bitcoin core to rescan all?