Is it possible to show messages from only one channel in the important news?

Yes, this is possible. As indicated earlier, you can indicate per channel what priority the messages in that channel should have. This gives you control over which messages are shown in important news. If you want messages from a channel to always be shown in important news, choose 'important' as the priority. If you don't want messages from other channels to be shown prominently, choose the prioritisation 'normal' or 'low'.

Only when there are no or too few recent posts in a channel prioritised as 'important', the posts from channels with a lower prioritisation will be shown.

For the settings, go to admin panel > channels > pencil of the desired channel > message priority and set one of the following prioritisations:

Note: It will take 1 hour for your changes to affect the overview.