You can’t do that, but since textures can’t store negative values anyway (unless HDR (float), and even then I think they’re abs values only) it probably doesn’t matter. Why not just put a clamp node after the texture sample when you read from it? You can use ClampMin Only if you don’t know what the max value will be.
Also, although you’re writing -0.1f to it - that’s just going to draw -0.1f directly, it won’t subtract anything from the existing value if that’s what you want to do?