I don't know any way and it would surprise me if there was since it's an aim of Markdown to get rid of bad markup.
What does justify text to be centered? Mostly you do this for some kind of header-text, use #, ##, etc. for this.
Maybe you want to use it as some kind of footer, use --- (hr) for the separation.
Centered text is just meant for esthetic reasons but the text doesn't really have a justification for this, so Markdown doesn't support such usage in order to keep a clean standard.
If it's images you want to be aligned centered I can understand your sense of esthetic. But you still need the admins help to allow this (e.g. using this hack).
