Adding Downloaded Custom Units

[73rdHighlanders]

I've started playing the BtS Earth 1790 AD scenario and decided to hunt around for some custom units to make it feel better for me. I've downloaded 11 Industrial Units, GrenadiersGalore and Napoleonic Dragoons. I love these depictions and they suit exactly what I was looking for but I can't figure out how to get them into this scenario.

If anyone feels compassionate enough to tell a noob how to get these in and working I'll send over a camel load of incense for your burning pleasure!!

TIA.