Unfortunately, I am not aware of any trick to get a 'real' leader on the map (MGL or SGL).
The problem is that SGLs are not accessable with the editor at all, and MGLs are defined as 'battle-spawned unit'. While you seem to be able to place a MGL on the map, it in fact is only a unit that looks like a MGL, but *does not* have the MGL ability (form Armies + rush structures).
That mock leader can only rush
wonders...
You can preplace an Army without problems.
It is a pretty sad bug IMHO, for scenario makers. If you really want an immediate MGL, the best chance would be to give the Palace the Heroic Epic ability, and start with sevreal Elite Blitz units and a stack of 1HP AI 0.1.0 units next to your start
.
Note the Napoleonic Europe Conquest was designed before the Conquest editor was altered, so it
may indeed by possible to start a map with the PtW editor and a 'Leader' (which should be transformed into an MGL in C3C), an finish it with the C3C editor.
I never tested it, though; and I never heard about some ever tried it.