Can I pour concrete walls between these structural posts?
You will need to dig/form a footer for your wall, appropriate in size and depth to support the wall itself and for frost heave in your area.

You could then form and pour, or build a block wall, between the posts. It will need to be reinforced (rebar) if you want it to last, and if stabilizing a slope it would need to be strong (tie-backs?), waterproofed for soil contact, and hydraulic considerations taken (gravel backfill, drains, etc.)