If you have a second generation v2.x firmware My Cloud, yes you can modify the firmware using SSH to install and run third party apps.
If you have a first gen v4.x firmware My Cloud, you cannot use the process used for the 2nd gen single bay My Cloud to add additional apps. The first gen firmware is different than the second gen firmware. And no one cannot upgrade a first gen to second gen firmware.
Use the forum search feature to find the few apps that can be installed to the first gen My Clouds.
For example see the following thread for various apps that may work on the first gen v4.x single bay My Cloud units.
