Hi,
To install the installer you need to:
- enable installation of untrusted software in settings app (sorry, I don’t know the exact name of the entry in English),
- download the installer RPM from OpenRepos,
- open the downloaded RPM and accept the installation.
Then you can disable the untrusted software again.
@jameson gave a good answer about presence of Storeman and its installer in the official store. In short, they are not allowed due to use of restricted APIs.