Basically the parity version of this.
Can Parity import raw private keys or Geth keystore files?
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I seem to remember just copying them into .local/share/io...../keys. The keys I had were generated on myetherwallet though, not geth. Or depending on how you installed, it might be ~/.parity/keys.
I think there is also an "import" function in the Parity UI that is accessible at port 8180. So open a browser to 127.0.0.1:8180 on a computer that is running Parity.)