I do this all of the time.
I take the size of my quilt and add the extra inches needed for my long armer. I write this down.
Then I take my extra blocks and put them on the floor (my design space), then figure out how much I need to add to the sides of the blocks.
Sometimes I sew the blocks together in a row/column, other times I place them in different spaces on the back.
It's kinda fun to see how it all comes together. I have never made a solid back. My girlfriend likes to call it a bonus quilt!