You cannot mod the summoned allies AFAIK

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You also can't mod new spells or new summoned monsters
Heroes-numbers can be found out in Edis unit_ID document

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Use this for clearing sites :
12.25 #clearsites
Clears all start sites for this nation.
Then feel free to add as many sites as you want with this command :
12.26 #startsite “site name”
Adds a start site for this nation. Table 15 shows some
suitable start sites.
All sitenames can be found in Edis Sites.xls .
You have unfortunately to chose the national heros only , so you can't get more then the nation gets . I can't make 9 Nazgul heros for my mordor mod e.g.

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And most evil you have to make unitgraphics . This is the part that takes longest . I am experimenting around there atm by testing various programs because paint is complicated because it can't save as targaformat afaik .
I can't draw neither but you should find with some patience somewhere fitting pictures of the new creatures you have in mind .