The Footer Columns sidebar is not really intended for this, and center alignment of content within one of the columns (no matter how many are used) is difficult.
You can of course, wrap your content inside a unique class, then attempt to use margins to align it, but there's no assurance it will work as the Footer Columns have their own CSS covering content alignment.
A more effective approach might be to use a different option, such as a Universal or Full-Width sidebar, placing your content inside a text widget, then, again, using the same class method, with margins. These will be unaffected by the Footer Column pre-set structure, so alignment will work.
This link will help explain the CSS in greater detail: