Under Inquisit 5, this is not possible with <page> elements. It would be possible under Inquisit 6 by using inline HTML markup ( cf. https://www.millisecond.com/support/docs/current/html/language/markup.htm ).
<center>This will only work under Inquisit <b>6</b>.</center>
/ preinstructions = (example_page)
With Inquisit 5, you instead need to present HTML files as instruction pages per <htmlpage>.
You can format your HTML files in any way you need, including centering any text they display.