Using ent_blendpose you can blend between two poses, but instead if blending them I would like to combine them, like when you use ent_animate with ANM_ADD. Has anyone ever done this? is it possible at all?

I can add ent_animate(me, "attack", my.ANIM, ANM_ADD); before or after e.g. ent_blendframe, but the animation will look a bit broken (atleast here). So that's probably not helping much, but maybe something to try out.

my idea is for example having shoot animation on pose 2 and use ent_bonereset to remove legs animation while on pose 1 using a walk animation and ent_bonereset to remove torso and arms animation. Later I would like to merge both poses so the character can walk and shoot at the same time.

The alternative is creating on med separated animations for the upper body or legs and just use ent_animate with anm_add

So if I understand correctly you already have a walk anim and an attack anim and want to combine those for an attack-walk anim, but the walk anim also moves torso and arms a bit (is that correct?). In that case how about;
1) ent_animate with walk anim.
2) reset the bones of arms and torso etc.
3) ent_animate ANM_ADD attack anim.

yes, but the attack anim has also a legs pose, so I can't just use ANM_ADD as it would broke the legs walking anim. If I modify the anim in med then I could be able to do that, but I would need to do it also for other upper body anims.

Hmm yeah that creates a challenge. A wild idea; get bone values from your blend pose and temporarily store them, than later add through ent_bonerotate (since it adds instead of sets). Dunno if it will work good enough but might worth a try.

but how could I get the bone values? I think ang_for_bone doesn't give the current bone angles but calculates the absolute angles in world space