Will adding more sugar during fermenting process increase the alcohol content?!
Answers: Yes but there's a limit. Yeasts can only function up to a certain alcohol percentage, and it's dependent on the strain of yeast you use. You can max out, and at that point adding more sugar won't do anything except make it sweeter.
no but adding more yeast will
Yes, adding more fermentables during fermentation will make it higher in alcohol. Adding more yeast won't, unless you just have a stuck fermentation. Yeast needs the sugar to make alcohol, so add more, and you'll get more.