I do not think there is any problem with asking or additional funding, as long as you are respectful in your request. One of my offers did not include health insurance, and the other did. I asked my POI if there was any way for the program to pick up the cost of my premium through the graduate student insurance plan, and she said that there was not, but that she would try to find a mechanism to add extra money to my stipend so that I can buy my own insurance on the healthcare exchange. The program director OKed it, although I am waiting for details on how that money will be dispersed. They have said it is complicated to add funds to standardized stipends, especially at state schools, but when there is a will, sometimes there ca be a way. Good luck!