You can't set a new unit to use an existing graphic. #copyspr mod command would work, but unless I completely misremember it wasn't added until Dominions 3.
Fans have extracted the tga files used in Dom3, and you can find them from
here. They are organized by both unit id number and name, so you can find the correct tga file even if you don't know the id number. You can find the id number by pressing Shift+i while viewing Knight of Avalon.
Once you find the correct .tga files, just put them into the mods folder and use #selectmonster and #spr1/#spr2.