In this approach, whole lines of poems by multiple authors are spliced together to create the new poem. The practice of borrowing can be traced to ancient poetry and the name itself comes from Greek and Latin terms for a patchwork leather cloak that was part of the Roman soldier's armor. At Wikipedia and on the Cento page of the Academy of American Poets you'll find much more information, interesting tidbits and surprises, and examples.
The problem for found haiku is that cento only comes into its own in longer poems that offer scope for the adapting poet's literary prowess and wit. Just when I was about to declare cento and haiku incompatible, I discovered "Book Spine Poetry". Here's an example of mine. It's not exactly a cento but it does use a different text for each line.
What's on your bookshelf?