This is more the seed of activities that can be devised using a common technique, than a worked-out activity.
I learned about "writing into" from John McGough, a TDU organizer and lover of poetry. He sent me this poem by Robert Kelly in which Kelly writes his own poem into Walt Whitman's "When I Heard the Learn'd Astronomer." Here's the first bit of Whitman's poem followed by Kelly's version:
WHEN I heard the learn’d astronomer;
When the proofs, the figures, were ranged in columns before me;