Can I make potatoes sweet by cooking them?
A raw potato is 79% water, 17% carbohydrates (of which 88% is starch) and 2% protein. So your question is essentially: can starch be turned into sugar? 

In theory the answer is clearly 'yes': this is what plants do when they break down the starch created as the final product of photosynthesis, and what happens in the human digestive system. Starch degradation is a complicated process involving several different amylase enzymes. 

In practice, I think the answer is 'no'. In a raw potato, starch is present in many small granules. When cooked, the starch gelatinizes: the granules burst and water is absorbed. This is a irreversible chemical change that obviously has a big effect on taste and texture. But it does not produce significant amounts of sugars. 

A closer match to the starch degradation process in plants occurs in brewing. When beer is brewed from barley by heating a 'mash': the natural amylases present in the grain convert starch to sugars; these are then fermented by yeast. The exceptionally sweet-toothed might try to mimic this process with potatoes, adding something like wheat, barley or sweet potato that has natural amylase. You could then boil the potato 'proto-beer' to extract the sugar.