So neither of those initial replies are what you are interested in. While transferring images to be stacked is fine, rather than live stacking, it is typically done once all the images have been taken along with what are known as calibration frames.
The other reply regarding focus stacking in an entirely different image processing technique that is not applicable to astrophotography. 

What you are looking for is software that aligns and combines images with an averaging algorithm. That also can apply calibration frames used to clean up noise and gradients. 
DeepSkyStacker and SIRIL are free. There are also more advanced programs such as Pixinsight and Astro Pixel Processor.
You will need to post-process your stacked images in a good photo editor such as Photoshop, or GIMP which is free. 

This is something you'll want to do on a desktop, not on a raspberry pi. 

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