The Sprite shaders offer an Emissive texture slot which can be used, you can see it in action at the Stretchyman skeleton in the Sprite Shaders example scene, and in multiple URP scenes that also use this asset.
The alternative would be to just have a separate slot with additive blend mode set in Spine.
Theoretically you could also manually edit the alpha channel of the PMA atlas texture after it has been exported, using your favourite image editor. Then change the alpha value of the emissive pixels to 0 to change it to an additive effect on the texture side. You just need to be sure that your image editor is not messing with the RGB values when you set alpha to 0, which e.g. Photoshop does in some versions when exporting as png. If that's a problem, just save it as .tga or any other format that preserves the RGB information. Note that this is not a recommended workflow, as you will need to reapply your work when re-exporting your skeleton.
