I'd like to add some spears to archer minis, like some haradrim. However, I'm struggling to find a way to attach the polearms to my warriors. Historical, polearms were carried like a walking stick. Any recommendations? I could attach them to the backs of my warriors but I can't any historical reference to sheathing a spear on one's back. I know LOTR is fiction but a root in realism adds weight to the legendarium. At least in my opinion.

How about doing it the other way around: attaching bows and quivers (from the Haradrim Raider set) to the back of Haradrim Warriors with spears? That will definitely look more realistic.
Another, more elaborate conversion, would be to convert a few Haradrim with spear such that they carry a bow in their left hand and wield their spear in their right hand.
I always find the just tacking on a spear to the back looks really weird and unrealistic. I mean, how is it going to stay put?
Putting a spear on the base would be better, I think, as if the Haradrim had just walked with it, stuck it into the ground and grabbed his bow. Or you could point the spear into the direction it is shooting, as if the Haradrim have made some improvised stakes.

I certainly plan to add some quivers to my spear wielding warriors, I'm just trying to think of ways to add spears to the bowmen

Add the spear to their back a green stuff a sling that wraps around their body.

I thought about that too, but no one in history did that. Again, I know LOTR isn't historical by any means, but I would like to do something that is practical and realistic.

The only way I could think of adding spears to bowmen is to have the bowman holding the spear with the hand that is holding the bow and the butt of the spear on the ground. It could be used as steadying the bow hand and the warrior could then easily drop the bow and use the spear in close combat.

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Many historical Middle Eastern armies had infantrymen armed with both spear and bow like the Sassanids and Persian Immortals. I would imagine that the spears were either stuck into the ground, stacked in a pyramid or just placed on the ground ready for use. Anything else and they would probably get in the way.