Migrate existing content types to MongoDB
Sure - you want to use the Migrate module to map your fields from a datasource (ex. CSV file) into your new MongoDB. Moshe has a post on this, more related to performance of his method for Amazon RDS but there is an example module that extends the Migrate API to pull the data from a CSV file into his MongoDB database. The relevant include file is here (maybe a little to long to include on stackoverflow?). 

To get your data out of the content-type into a csv dump, I would recommend using Views Data Export.