Yeah, sadly that option is only available if the list specifies it. And there are very few: Camillan Romans can use it to disguise Sp (I) as Sp (S); Parthians can use it to disguise Kn (X) as LH (F); and in book 4 Cilician Armenians can use it to disguise LH (F) as LH (S). There are probably a couple of others but I can't think of them.
(The last is particularly pointless because the LH are disguised as a troop type that isn't otherwise available in the army, so as soon as you plonk a pair of Mongol LH (S) elements on the table your opponent will automatically know they're actually LH (F)...)
But all isn't lost! It's good that you're looking at the stratagems for what you can do to hoodwink your opponent, as well used stratagems add another dimension to armies. A useful combination of stratagems is Exaggerated Numbers and Ambush - use the first stratagem to bulk out the visible part of your army to disguise the fact you have an ambush or two. And personally I'm rather partial to the Delay Battle stratagem for its ability to improve the chance of your off-board command(s) arriving.
Very early on in the life of DBMM I remember reading a post from a player involved in a game between Early Germans and Marian Romans. He was the Roman, and he could tell there was a German command missing. But he didn't know whether it was in ambush somewhere on the terrain-covered table, on a flank march (possible on either flank), or a delayed command. He didn't have the resources to go check every possible ambush site, nor could he spare many troops to guard both flanks against the possible arrival of a flank march. But he also wasn't keen to risk a quick attack on the enemy he could see, for fear of the innopportune arrival of the missing Germans, especially if they were delayed.
That's a lot of indecision caused by one command which wasn't visible, and that's the sort of indecision one or two well-chosen stratagems can cause. So keep looking and trying things out, to see what nasty (that is, neat) tricks you can pull on your opponents with stratagems.
