Can Splenda be substituted for regular sugar when making your own sweetened condensed milk?
The answer is yes.  I found this link for you from Genius Kitchen 
https://www.geniuskitchen.com/recipe/sweetened-condensed-milk-with-splenda-412496

There are many recipes for both low carb and diabetic friendly Sweetened condensed milk.

Also the following:  https://www.yummly.com/recipes/sweetened-condensed-milk-with-splenda