defaults write to child property

[neuralstatic]

i can't find this online anywhere.

i need to set a preference property for all users via a command line.
unfortunately it's a property that is a child of another

it's for com.apple.mail.plist, and this approximates how it would look in the plist editor
MailAccounts
__1
____CachePolicy String OFF


defaults write com.fake.fake MailAccounts '{ 1 = { CachePolicy = OFF; }; }'
this works b ut wipes out other entries under the "1" child.

i think i need to kill the "=" sign, but i can't get the syntax.