Standard enthalpy of Al2(SO4)3 is -3441.04 KJ/mol and Na2SO4 is -1387 KJ/mol.

I want to sum these two things such as Na2SO4Al2(SO4)3.

And Standard Enthalpy of Na2SO4Al2(SO4)3 is (-3441.04)+(-1387) = -4828.04 KJ/mol, is it right?

Hm.. standard enthalpy on its own is the "energy released by" at standard conditions... I think you meant the standard enthalpy of formation?

But no, when you form one mole of Na2SO4, it will release 1387 kJ of energy, and when you form one mole of Al2(SO4)3, it will release -3441.04 kJ of energy. If you 'merge' them together to make a new compound, remember that bond formation is exothermic and assuming that it's successful, it will release much more than 4828.04 kJ of energy.