You can with prosilver, Bl@ck is prosilver just modified width and colors with borders. Many have done it with prosilver. Also Bl@ck's pending 1.0.7 update increases the width to 85% of the page. Here a simple google search got me these results:
http://www.phpbb.com/community/viewtopi ... #p10382385
http://www.phpbb.com/kb/article/adverti ... in-phpbb3/
http://startrekguide.com/community/f27t ... _post.html
Again, i am happy to help with style specific problems, but since i don't have ads myself i can't be of any help further than that. You are going to need knowledge of html. I would do the sidebar mod, than put your ads in the sidebar.
A better idea would be to put ads between posts in threads, or even better above the forum, but below the header like in the link i gave you, small word only ads would be better. Sidebar ads are 1998 and tacky from a design point of view. Keep in mind many run adblock or other adblocking software, so if you can live without ads, it would be a better experience for the user. At least until your site starts gaining members/visits. Again, that is my opinion about the ads, but i think a different option would be better than to have sidebar ads.
As long as you find a solution for a prosilver based style, it should also work for Bl@ck. The edits done to Bl@ck are minimal, so if those links can't help you, keep searching google.