how would i add the member rank to a sig

[SEOvB]

Are you sure thats a rank and not just a postbit background?

You can do something like

.posts {
        background: #EEEEEE;
        background-image: url(path/to/image.png);
        background-repeat: no-repeat;
        background-position: bottom right;
}

Then modify the postbit(_legacy) area to include the posts class for the message area.

If you want the custom ranks to appear over there, you'll have to search the postbit template for the ranks code and use a floating div probably to set it

<div style="float:right; padding: 3px;">
Rank code
</div>