The immutable hierarchy doesn’t contain a MultiMap, so you won’t be able to use the converted structure with the same convenient syntax. But if you’re happy to deal with key/valueset pairs, then:
If you just want a mutable HashMap, you can just use x.toMap in 2.8 or collection.immutable.Map(x.toList: _*) in 2.7.
But if you want the whole structure to be immutable–including the underlying set!–then you have to do more: you need to convert the sets along the way. In 2.8:
x.map(kv => (kv._1,kv._2.toSet)).toMap
In 2.7:
collection.immutable.Map(
x.map(kv => (kv._1,collection.immutable.Set(kv._2.toList: _*))).toList: _*
)