<<HighEnergyPhysics`FeynCalc` ;
DeclareNonCommutative[s];
FI;
ssimp = ( 
      ( Expand[DotSimplify[#, DotSimplifyRelations -> 
            {s[i_].s[j_] :> (k=Unique["k"]; KroneckerDelta[i, j] + I Eps[i,j,k] s[k])}]
              ] 
      ) //. (KroneckerDelta[x_, y_] f_ :> (f/.x->y) /; !FreeQ[f, x]) /.
      ( x_ s[ij_] :> ( (x /. ij :> k0) s[k0] ) /; !FreeQ[x,ij]) 
        )&;

ssimp[ s[i1].s[i2].s[i3].s[i4].s[i5] ]

