Hi !
Since you have backup file (adi) and you know the missing qso one solution is use console commands.

First check that you find the right qso from adif. It has to have something unique. I assume here it is callsign, but it can be date, mode , time what ever that any other qso does not have.

Assume your backup is in file /tmp/hi.ADI and call you are looking is LA3SP

Check it is there give command:
cat /tmp/hi.ADI | grep LA3SP

Should give something similar like:
<QSO_DATE:8>20180222<TIME_ON:4>1618<TIME_OFF:4>1619<CALL:5>LA3SP<MODE:3>FT8<FREQ:5>5.359<BAND:3>60M<RST_SENT:3>+16<RST_RCVD:3>+06<NAME:6>Bjørn<QTH:5>Hovet<QSL_SENT:1>N<QSL_RCVD:1>N<GRIDSQUARE:4>JP40<MY_GRIDSQUARE:6>KP01TN

If that is ok you can make file /tmp/ha.adi having that one qso:
echo '<EOH>' > /tmp/ha.adi; cat hi.ADI | grep LA3SP >> /tmp/ha.adi;echo '<EOR>' >> /tmp/ha.adi

And the import /tmp/ha.adi in usual way to cqrlog.

Just change filenames and call to what ever you need.

If same call has several qsos, but only one is missing from log then first do the same, but before import to cqrlog manually remove
unneeded lines (they cannot be so many any more) from /tmp/ha.adi with you favorite text editor.
Remember that every line has corresponding <EOR> (end of record). In case you made /tmp/ha.adi there is only one EOR so it is assumed there is only one dataline, too.

If you want to add more lines than one add also same amount of <EOR> at the and of every qso line.

This is one way to do it.
__
Saku
OH1KH