Solution:
In order to import your old posts into the new one, you have several options, depending on your current application.
Custom Importer
You could write a custom importer script. To get started you would simply connect to the database of your current application and query
all the posts to import. Then you would need to bring the data
into the right format and import it into the new WordPress database.
This is the way I recommend you if you have thousands of posts and maybe some custom fields and values, which need to get mapped to new fields.
The downside of this approach is obviously the time it takes. Especially when it comes to files, relation tables etc.
Third-Party Importer
Maybe you are already using a popular blog platform. In this case, you will surely find several importer scripts or plugins.
For example a Wix to WordPress Plugin: https://de.wordpress.org/plugins/cms2cms-wix-to-wp-migrator/
If you have some basic blog posts (no custom fields, etc.) this is definitely the way to go since it saves you a lot of time.
RSS Importer
If your blog’s using RSS
you can make use of an RSS Importer. The RSS Post Importer plugin fetches an RSS feed and publishes the full article content of each Feed Item as a stand-alone post.
Check out this one: https://de.wordpress.org/plugins/rss-post-importer/
Manual Import
Sometimes none of the above options works out for you. In this case, you would probably need to import them manually by copy-pasting them into your system.
Especially when you don’t have too many posts this is probably the fastest way to go.