You can write one for Map
using recursion:
Map.metaClass.addNested = { Map rhs ->
def lhs = delegate
rhs.each { k, v -> lhs[k] = lhs[k] in Map ? lhs[k].addNested(v) : v }
lhs
}
def map1 = [
"a" : "1",
"animals" : ["cat" : "blue"]
]
def map2 = [
"b" : 2,
"animals" : ["dog" : "red"]
]
assert map1.addNested( map2 ) == [
a: '1',
animals: [cat: 'blue', dog: 'red'],
b: 2
]