It is insoluble in H2O. May be you are trying to insert NiS particles in your hydrogel. If you add HNO3 to the mixture, NIS can be solubilized in the gel. Its mean you are impregnating Ni2+ ions, not NiS, which may also favor more gelations of the polymers. However, there is enough room to distort/hydrolyze the gel depending on network-density and structure of the gel.
Yes it should be feasible as the alginate polymer contains uniform negatively charged groups that will interact with the positively charged Ni2+ ions bringing an organised and uniform final structure.