I like that idea too, but I don't know how to do it. It should be strategic points, like having abundant resource potential or a prime location, a strait or a plateau, for example.
There are several ways to make Hamippoi work. I opted to make them all 1 actor and unit. Other ways could be:
Cavalry can train cheap melee footmen called "Hamippos." Either as a default feature or unlocked with a tech.
A tech called "Hamippoi" which boosts cavalry.
A "Hamippos" infantry unit with a strong aura benefiting cavalry.
Mix and match any of the above.
I went with the one unit option for this release. Next release I could try something else.