One approach that might work in writing a poem based off of the novel is to compose it from a particular character's point of view. For example, a poem from George, talking about what he did and how he views it, could be quite powerful. I think that being able to express in a poetic form George's emotional state could lend itself to being compelling work. The poem could enter George's mind and probe into the implications of what he did and having to live with the consequences of such actions. The poetry of lonliness can be seen in this regard.
In a similar manner, entering into Candy's mind could also reveal a very sad poem. Candy, finally broken in recognizing that his dream is destroyed with Curley's wife's lifeless body, could prove to be powerful. Candy who is alone without his dog and devoid of his dreams could be a subject area where personal feelings of regret, anguish, and a sense of loss could emerge. This might be where another poem that enters into the frame of reference of a particular individual in Steinbeck's work could be worthwhile. Being able to write a poem based off of Steinbeck's work might function best if it is focused on a particular character and how they view being in the world as a result of their characterizations offered.