You can bind a specific mipmap level for imageStore access with setShaderInput:
nodePath.setShaderInput("name", texture, read=False, write=True, z=-1, n=0)
where n is the mipmap level index and z is the layer/page index (eg. of cube map), where -1 means to bind all layers.
Panda will generate mipmaps automatically when either (1) uploading new texture data and some mipmap levels are missing, or (2) you are rendering a texture with mipmapping enabled. It will not automatically do so if you write to it using imageStore.