Comment 3 for bug 162185

As mentioned in bug #33065 (and just confirmed by me) this requested feature is included in Synaptic, but under a different menu item.
Procedure:
In Status grouping select "Installed" packages
Select all packages (Ctrl-A)
In menu File -> Save Markings
In the pop-up be sure do select the tick-box next to "Save full state, not only changes", then save.

Done, import via File -> Read Markings

Could you please confirm that this procedure works for you?