Yes, you can ad Vusers during a running test in Performance Center. You can add Vuser groups, or you can modify existing groups during the test run.

To add a group. On the Performance Test Run page, click Design Groups and Scheduler. Then select Add Groups. The Add/Edit Vuser Group dialog box opens, enabling you to add a Vuser group.

To edit a group. On the Performance Test Run page, on the Groups pane, place the mouse cursor over the name of the group you want to edit. Click the down arrow that appears adjacent to the name, and select Edit Group. The Edit Group dialog box opens, enabling you to edit the group's details