Certificates stored in Octopus can be exported (if you have proper access). Navigate to Library and in the right side pane, click on Certificates. Select the certificate you want to export. Click on the three dots:
If you know the original certificate password (if there is one), you can select Original and click OK. Otherwise, you may want to export as PFX (PKCS#12) and define your own password.
PROTIP: In the past clicking OK to export the certificate did nothing unless you were using Internet Explorer.