You can dissolve elementary sulfur in water solution of (NH4)2S for preparation of ammonium polisulfide , but some traces of ammonium sulfite and thiosulfate would be formed as follows:
S2-+ H2O= HS- + OH-
OH- + S = S2- + SO32- +H2O
SO32- + S= (S2O3)2-
Rate of the mentioned above reactions of disproportionation of sulfur in alkali are depended on concentration of OH- and seems to be negligble in water solutions of ammonium sulfide.