but will future versions of KM come with support for XPIs?
Probably not, because it would involve bulking KM with more XUL which ultimately slows it down, but you never know.
And is there any way I can get the adblock extention for FF/Mozilla to work with K-Meleon? Or is there anything similiar for KM?
Not for individual sites. The only alternative for now would be to use a general approach such as an altered Windows hosts file or an external app or CSS. View the Block Ads section on the following page.
http://kmeleon.sourceforge.net/wiki/index.php?id=ControllingDisplay