Well youd have to first create the roman texture with photoshop.

If you dont mind that the units use the carthaginian texture (so they look exactly like the carthaginian unit) then you can just copy the carthaginian entry and change the faction to 'romans_julii, romans brutii, romans_scipii, romans_senate'.

If you search around the internet im sure you could somehwere to dload a roman phalanx unit with a skin and unit card and everything already made.