nPara = 3;
cf = Compile[{{x, _Real, 1}},
  Sum[x[[i]], {i, nPara}],
  CompilationOptions -> {"InlineExternalDefinitions" -> True}
  ]

With[{nPara = nPara},
 Compile[{{x, _Real, 1}},
  Sum[x[[i]], {i, nPara}],
  CompilationOptions -> {"InlineExternalDefinitions" -> True}
  ]
 ]