I use satellite receiver based on Debian. Is any possibility to create mount point (if no exist) for filesystem by appropriate entry in /etc/fstab ? I mean about situation when during update/upgrade procedure, most of system is overwrite by new version. But in this case system configuration is restored. That is most of /etc dir, including fstab too. So it would be nice to create such dir via fstab. The content of this filesystem is necessary during boot procedure so creating/mounting from script after boot is too late. Is it possible?

If, after creating your directory & mounting it via fstab, & after a reboot, it is no longer there, you will need to remaster your distro.

If it is a normal distro, an update & upgrade should not lose it.

Yes, "ordinary" update/upgrade is possible. The unit is Debian based but has own repos, dedicated for such devices. But the problem is mainly when any changes in kernel are made. In this case practically all image is installed from beginning and /etc located config files are restored. Its because there is no any kernel tools installed in original image. In such cases /opt dir is lost and it is needed to create one manually. /opt is use to mount SD card with additional commands (entware) not available from original repo. I want to automate whole process and run it from cron so I need to create mount point /opt if not exist, during mounting filesys from fstab. Thats my problem.