texture painting over an existing texture

From the shader editor open the texture in the sidebar

Add a diffuse shader OR a principled NSDF, and mix it with the current texture output using a mix shader node. Select diffuse node and press ctrl+t (only with node wrangler add-on enabled). This adds a texture node, mapping node and texture coordinate node.

From the image texture node create a new image. Give it a name and resolution. Image texture alpha to 0.

Set the alpha output to the mix shader factor node.

You may need to UV unwrap or smart uv project at this point in the EV Editor tab.

Now go to the texture paint tab. Go wild!