With a little bit reverse engineering I found a way how to add more applications on the uHome desktop:

Note: The preinstalled applications are stored in /usr/share/click/preinstalled and "after" installed applications are in /opt/click.ubuntu.com
As someone mentioned here, using a command lomiri-url-dispatcher-dump you can check applications with existing links.

You need to choose one of those applications (e.g. Weather) and go to its installations directory (/usr/share/click/preinstalled/weather.ubports/6.0.2). Now find the file named lomiri-weather-app.url-dispatcher
So in other words the goal is to find some existing *.url-dispatcher file as an example. I found a second option used by uHome application itself and that is a json file instead.

Now do a copy of this file into your home directory. Rename according your new application, (e.g. level.luksus42.url-dispatcher). Edit this file and rewrite the url to desired word. For example rename "weather" to "level".

After this you need to make the root filesystem writable (sudo mount -o remount,rw / and copy the file to corresponding directory for the new application (e.g. /opt/click.ubuntu.com/level.luksus42/0.5/level.luksus42.url-dispatcher

Next step is to edit the manifest file of the new application (e.g. /opt/click.ubuntu.com/level.luksus42/0.5/.click/info/level.luksus42.manifest). Find the section "Hooks" and add a line refering to your "url-dispatcher" file.
Example:

    "hooks": {
        "level.luksus42": {
            "apparmor": "level.luksus42.apparmor",
            "desktop": "level.luksus42.desktop",
            "urls": "level.luksus42.url-dispatcher"
        }
    },

Be carefull about the commas at the end of lines. Note, that each line has comma at the end but the last line doesn't.

Now reboot your device and check if the new link was added lomiri-url-dispatcher-dump. If you see it there you almost won. Now you need just to add a link onto uHome desktop, use the chosen url and search the app directory for corresponding icon.

I will be happy if this guide helps someone except me. And I wish, that the uHome app will be migrated to the current and future systems. I also would appreciate if you add an option to group Links in folders like it is in android/iOs systems.

Thank you @pavelprosto for this app.

Thank You. I contacted them via email and got a reply, that the update will be available soon. However it takes already few weeks since the reply. Might be I just have to be more patient.

I successfully installed UT as second OS on Volla Phone X23 (on SD Card). The Volla Boot Manager installed the version OTA-4. In the meantime a version OTA-5 was released. I found 3 ways how to update the OS to the newest version. However none of those worked:

• the update option in Volla Boot Manager shows, that there is no update.
• the Update option in UT System Settings offers to install an update, but this just reboots the phone to Recovery mode and nothing happens.
• I found a guide to use ADB tools and command "sudo system-image-cli -v -p 0 --progress dots" but this also just rebooted into Recovery mode without further action

Can someone advice please how to update the UT running as a second OS?
Might be remounting the root filesystem into write mode and run apt update sequence? Is this a recommended way?

Thank you for any advice.