Apparently it’s pretty easy, but there are some pitfalls. The first step is to export the certificate as a DER encoded X.509 certificate. This can be done using Firefox (on a PC) by clicking on the SSL’s lock sign in the address bar, More Information -> View Certificate -> Details -> Export. The exported certificate needs to be saved on the root directory of the internal storage of the phone, with
*.cer extension (or
*.crt). Other extensions will not work.
Afterwards, on the phone, click on “Install from device storage” under Settings->Security->Credential Storage. If you did everything as you should at the previous step, it will display the certificate name, and ask you to confirm its installation. If you’ve exported the certificate as the wrong format, gave it the wrong extension or placed it somewhere else than the root of the internal storage, it will display the following error:
No certificate file found in USB storage
If you see it, just make sure you are exporting the certificate correctly and saving it at the right place.
More details: Work with certificates (Geared towards Galaxy Nexus, but should apply to any Android 4.0 and above.
Updated Aug 2015: Fixed a broken link.