You might adapt something like this:
MakeBoxes[Sum[e_, {i_, a_, b_, 2}], TraditionalForm] :=
RowBox[{UnderoverscriptBox["\[Sum]",
RowBox[{MakeBoxes[i == a, TraditionalForm], ",\[VeryThinSpace]",
MakeBoxes[i, TraditionalForm], " even"}],
MakeBoxes[b, TraditionalForm]], MakeBoxes[e, TraditionalForm]}]
MakeBoxes[Sum[e_, {i_, a_, b_, m_}], TraditionalForm] :=
RowBox[{UnderoverscriptBox["\[Sum]",
RowBox[{MakeBoxes[i == a, TraditionalForm], " (mod ",
MakeBoxes[m, TraditionalForm], ")"}], MakeBoxes[b, TraditionalForm]],
MakeBoxes[e, TraditionalForm]}]
Sum[f[i], {i, 0, n, 2}] // TeXForm
(* --> \sum _{i=0\text{,$\, $}i\text{ even}}^n f(i) *)
Sum[f[i], {i, 0, n, 3}] // TeXForm
(* --> \sum _{i=0\text{ (mod }3)}^n f(i) *)