I don't think you can. The best I can think of would be an event to measure the size of the form, determine where "roughly" center would be, then add leading spaces to the front of the caption to move it to the middle.

You can, with a trick: add leading spaces to the caption.  

If the form is to be resized, you need to use VBA (in the form's Resize event) to keep calculating the right number leading spaces to add (see pictures below).  A bit of trial-and-error may be needed since alphabets and spaces have different widths.

Caveat: for some reason only the first 128 characters of the caption are visible; anything longer is cut off.  If you have a long caption on a large form that seems large enough to accommodate a long caption, the caption won't be centered because it will be cut off, leaving a large space on the right (see picture below).

Another way is to make your form borderless (set form's Border Style to None).  That gets rid of the form's caption bar.  You then create a fake caption bar with a *textbox* (see picture below), for which you can use many text formats.  But then you also need to create your own maximize/minimize and close buttons, and re-create all their behavior with VBA.  You also need additional VBA to resize and move the form.  This gives you the most freedom, but of course (highly) impractical for most users.