In BeerSmith 2 it doesn't look like you can do exactly what's described above, so here's how I approached it:
In the "Grain, Extract or Malt" popup from a recipe, or the Ingredients > Grain tab you can configure the bitterness of hopped extract.
However, the value has to be entered in "IBU-gal/lb", which is described as "IBU content for hopped extracts expressed as the number of IBUs in a pound of extract dissolved in a gallon of wort"
I couldn't be bothered thinking too hard about how to convert this to the 28.8 IBUs that I knew the 1.7kg tin of Coopers Lager concentrate that I wanted to enter contributed to 23 litres.
I'd already created the Coopers Lager concentrate as an ingredient, so I created a new 23-litre recipe and added only 1.7kg of Coopers Lager.
Then I adjusted the "IBU-gal/lb" figure until the bitterness came out at 28.8. The figure I had to put in was 46.7.
So I then adjusted the Coopers Lager ingredient to this value, so that I wouldn't have to do it all over again next time.
This is probably as clear as mud, so let me know if you've got any questions. Or you reckon my thinking is flawed
Cheers,
Oliver