Can I assign submitted contributions manually to reviewers

We want to assign submissions manually to reviewers, i.e. papers submitted for a special part of our conference.

We still want to use the automatic assignment process of papers in a second step. Is that possible?

You can either assign submissions to reviewers or reviewers to submissions manually at any given time, usually after the submission phase is over.

Please proceed as described here:
How to Assign Reviews in ConfTool

You can either carry out the manual assignment before or after any additional automatic assignment.
You will find more information about the automatic assignment process here:
Automatic review assignments: How does it work?