MongoDB isn't built to work on top of HDFS and it's not really necessary since Mongo already has its own approach for scaling horizontally and working with data stored across multiple machines.
A better approach if you need to work with MongoDB and Hadoop is to use MongoDB as the source of your data but process everything in Hadoop (which will use HDFS for any temporary storage). Once your done processing the data you can write it back to MongoDB, S3, or wherever you want.
I wrote a blog post that goes into a little more details about how you can work with Mongo and Hadoop here: http://blog.mortardata.com/post/43080668046/mongodb-hadoop-why-how