Basement drain to sewer - how best to fill it in?

I have an unfinished basement. The walls and floors of the basement are concrete. I have a drain in the basement floor that feeds directly into the sewer. I know that my sewer line runs below the level of the basement (Roto Rooter has been a frequent visitor) to the main city sewer line in the alley. It is a gravity system; I have no pumps controlling my waste water. 99% of the time, waste water leaves my property as the designed. (My entire sewer line to the alley has been replaced, sometime in the last 10 yrs).

Last night, the sewer backflowed into my basement...again (this is the fourth time this has happened; the last was exactly one week ago). Roto Rooter snaked my line and found nothing. I called the City, and they know of no issues in my neighborhood. My neighbors have had no issues.

The backflow problems always occur at night when the house is using little-to-no water - no showers, no toilet, no washer, no disherwasher is running. I'm relatively sure I do not have a back-flow restrictor valve/flap installed (old house?). Every time this happens, the walls are all entirely dry, and the floor is wet, esp around the drain. I live in the desert - our ground is very dry, so I know it is coming from the drain.

I want to fill the drain hole up w/ cement (and install a pump for code purposes). How do I do this? Do I just mix concrete and pour it down?