There is no way to force activities to be critical in Primavera. They become critical based on durations and logic after the project is scheduled and starts to progress.
Only other way is to define the “critical” criteria…
Set the float time for identifying critical activities
You can specify a maximum activity float time for identifying critical activities. If an activity's float time is less than or equal to the time period you specify, the activity is automatically marked as critical.
Choose Enterprise, Projects and display Project Details.
Click the Settings tab.
Choose Total Float less than or equal to in the Critical Activities section. Type the value for which critical activities should have float less than or equal to, followed by the corresponding time unit abbreviation.
The available time units are minutes, hours, days, weeks, months, and years. (Your system administrator sets the abbreviation for each time unit in the Admin Preferences dialog box.)