only if they are used on the map, battle ones won't work, unfortunately (you can hack in +- new mechanics onto existing ones with workarounds, nothing more)
So if in the types.xml file
after the code:
<Item>
<Name>SPELL_EFFECT_FIRST_AID_TENT_PLAGUE</Name>
<Value>352</Value>
</Item>add something like: <Item>
<Name>SPELL_NEW_SAMPLE_SPELL</Name>
<Value>353</Value>
</Item>
And of course increase the total number of spells in the .exe - it would be useless?!

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Неудачное планирование ведёт к неудаче ...
NCF - мод, добавляющий дополнительных существ в HeroesV 3.1