It depends how much you use really.
I always look at slin as giving your pancreas a break or atleast aiding it.
If you use large amounts, I believe there would be no need for your pancreas to produce anymore. However it all depends on what type of insulin with the peaks and how long it’s in your body.
For example, using a fast acting slin. The amount injected may be enough that your pancreas doesn't need to produce anymore, but when that slin is out your system your pancreas would then kick back in