It is possible, though complex in RTW, to script that some units appear when a town is sieged, to represent untrained population coming to assist in fending off the siege. As bovi said, some mods have this scripting. The problem is that the new units don't just disappear when the siege is repulsed, and you have a bunch of 'free' units sitting around. EB has decided not to do such scripts at this time.
As far as just automatic units that appear and disappear when a settlement is sieged, based on loyalty and population amount, you'll have to ask CA for that in the next Total War.
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