Hi,
What exactly do you mean by "store" them? If you don't provide a Mapping elasticsearch will try to infer the field type of fields it doesn't know yet. Values like 0/1/true/false are treated as "boolean" if not explicitely specified. Regardless, if you index new documents with that said field, the "_source" of that document will always reflect the original JSON you used at index time, so if you have indexed a boolean field as "foo" : 1, "foo": "1" or "foo": true, this will always reflect in the docs "_source". Under the hood, booleans are stored as 1-char bytes in Lucene, so nothing to do there.
Or do you want to store them as Strings for some other reason?