To avoid having a lot of keypairs accross your different platform (ie AWS + Eucalyptus), you can import you existing public key to create a keypair.
Via the GUI
You can add a keypair via the Eucalyptus Console following these steps.
1 . Get logged in with your account/user credentials, then go in "Network and Security" -> Key Pairs
Once you're in the keypairs page, just click on "Import public key"
Once the upload is finished, you can use your private key to get connected to the instances.
Via the CLI
You can also use the CLI tool to upload your existing keypair.
euca-import-keypair -f <public key file> keypair-name