As far as I am aware bulk loading isn't supported via the SDK for .NET (it isn't for Java so I'll assume they are 99% the same).
However they do have a tool for bulk loading in json called the cbdocloader tool. This allows you to bulk load in json to a specific node and bucket of Couchbase. There is an official blog post covering the use of it:
http://blog.couchbase.com/loading-json-data-couchbase
Option 2:
You can always just use the standard set method, iterate over wherever your documents are (I'm assuming they are on the file system) and set each one individually.