First, I think you have to overwrite the entire unit (not just the stuff you want to add, copy the whole unit's xml to your mod file, then add what you want).
Second, I don't think charm monster will work because it will be dead at that point. But, you can have the same monster type join the army (although I havent tested it out)
Third, you forgot the treasure tag (looking at the core XMLs arent very helpful sometimes)
<Treasure>
<Title>Spider Join</Title>
<Liklihood>1</Liklihood>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>UnitJoinArmy</Attribute>
<StrVal>GiantBlackWidow</StrVal>
<UnitClasS>GiantBlackWidow</UnitClasS>
</GameModifier>
</Treasure>