According to the documentation
New in version 2.6: If you do not specify --collection, mongoimport
takes the collection name from the input filename. MongoDB omits the
extension of the file from the collection name, if the input file has
an extension.
So it seems it is supposed to do import one collection at a time. So unless you write a shell script to do it, I don't see a way.
mongodump and mongorestore are much better for taking full db dump and restore it at once as the other part of the same document says
Warning
Avoid using mongoimport and mongoexport for full instance production
backups. They do not reliably preserve all rich BSON data types,
because JSON can only represent a subset of the types supported by
BSON. Use mongodump and mongorestore as described in MongoDB Backup
Methods for this kind of functionality.