I managed to install a hosted port on my Surfans F20 with the firmware version 2.7. Finding out how to do it took some time. Maybe posting this will help the next one with the same problem.
1. I couldn't find any stock software. Therefore I could not use the Rockbox utility to install Rockbox. It had to be a manual install.
2. For that I used
3. I connected the Surfans to my computer. It is recognized as a USB-storage unit.
4. I renamed the file F20-v27-patched.upt to update.upt and copied it into the root directory of the Surfans F20
5. I unzipped the Rockbox file. It contains a folder named .rockbox. I copied this folder (not its contents!) into the root directory of the Surfans F20
5. I ejected the Surfans F20 and booted it. I made a firmware update (System Settings - Firmware Update- "Please confirm whether the firmware is updated" -YES). Now Rockbox is installed.
6. After rebooting the F20 I got a boot menu with Rockbox, original firmware and tools. Rockbox booted without any problem, and could be used.
7. Now the Surfans F20 cooperates with the Rockbox Utility. It's possible to install new builds, themes and fonts.
8. For now there is no Rockbox Manual for the Surfans F20. I used the manual for AGPtek Rocker to understand the settings.