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I don't know how high your stairs will be but consider this: When you pour them, you can figure that the pressure at the bottom from the concrete pushing out on the forms will be around 150 pounds per square foot, for each foot of height. The actual pressure depends on a lot of things but you can use this as a general guide. That sort of pressure will not be kind to any gaps that you might fill with foam.
I'd remove a section of the wall, pour the stairs, then build the wall back on either side. Or, build the stairs out of rock, mortared in such a way as to look like a dry stack wall.
JR
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