Nope, I don't think you can. Perhaps indirectly one could do it by altering unit stats etc in a way that the AI would chose a specific unit over the others, but since the choice 'algorithm' is not known, you woldn't know what to change. Plus making alterations would damage unit balance.

If you are that interested in having certain units gifted to the player, you could also do it with much more precision and no randomness involved - with scripts (I can see Dol executing me for high treason). The latest release of RTR, The Iberian Conflict, has a form of scripted "missions" that could help you. That is *if* you are not against using scripts.