If you click on the 'i' button to the right of the Description field, you can use this notes field to enter cheque numbers, or anything else for that matter. This will then appear in the notes column. Simples!
Attached is a screen shot to illustrate.
I checked Squirrel Prefs and the View menu to see if there is a way to make "notes" show in a column, only when an individual entry is viewed and then one clicks the "i" button.