stikkanimate yazdı

I know it doesn't work with Spine 3.x - we'd need to duplicate a character and set it to a different blending mode in Unity to make it work..

Please note that using e.g. spine-unity 3.X you can already override materials per Slot at runtime, and thus change from a Material with an additive shader to one with a multiply shader. You can achieve this in multiple ways, as described here:
spine-unity Runtime Documentation: SkeletonRendererCustomMaterials
spine-unity Runtime Documentation: Changing Materials Per Instance

If you mean that you would like to change the blend mode from Normal to Additive using PMA workflow and not having a second Material, then this is unfortunately not yet possible on any of the spine runtimes, neither in 3.X nor 4.0. The blend mode is by design set at the shared SlotData object and not at each skeleton's Slot.