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I just want to note that you are going in the right direction using the NewsManager and ContentManager classes. The News Module is a Generic Content based module and working with its manager class is very similar as with the Generic Content manager.
The Blogs Module is a Generic Content based module too, and has its functionality derived from the base module as well as the News Module. That means that working with BlogManager class to import Blogs is similar to working with the NewsManager. For more information data-related operations for Blogs, please take a look at the BlogManager API
Forums on the other hand are not based on Generic Content. However working with data and Forums is not very difficult. The Forums Module has its own ForumManager class, that can be used to import data. You can look at ForumManager Class Members
for more information on which methods you will need to use.
I hope that you will find this information useful for your needs to import Blogs and Forums. Plase feel free to contact us if you have more questions.
the Telerik team