See this: Blogger Help: Import and export blogs
To make a copy of your blog's posts and comments, you have to export your blog and then import it to another existing blog.
Export Your blog
To export your blog, simply click "Export Blog" from the Settings |
Then, click the Export Blog button. Your blog will be stored as a
Blogger export file (.xml) file which can be kept as a backup on your
hard drive or imported into another blog. Exported blogs are not
deleted from your dashboard or from Blogger.com.
Import Into an Existing Blog
To import posts and comments into an already existing blog, go to
Settings | Other and click on "Import blog" from the "Blog tools" section. Don't worry -- your template won't be affected by imported
content. Next, choose a Blogger export file (.xml) from your hard
drive and fill out the word verification beneath. By default, all of
your imported posts will remain unpublished until you publish them
from your dashboard later. However, if you would rather have all posts
published immediately, make sure to check the box next to
Automatically publish all imported posts before you click Import Blog.
If you didn't choose to have your posts publish automatically, you'll
need to manually publish them from the Posts | Imported tab before
they will appear on the web. You can either publish all the posts at
once by clicking the checkbox at the top and selecting all before
clicking Publish, or simply checking the ones you want to publish
one by one.
Imports and exports are only for posts and comments. If you want to save a copy of your template, click the Backup/Restore button on the Template tab.