like that?

import
  lua


proc NimMain() {.importc.}

proc get_hello*(state: PState): cint {.cdecl, exportc, dynlib.} =
  state.pushstring("Hello!")

  return 1

let
  test* = [
    Treg(name: "get_hello", `func`: get_hello),
    Treg(name: "", `func`: nil)
  ]

proc luaopen_test*(state: PState): cint {.cdecl, exportc, dynlib.} =
  NimMain()
  state.openlib(cstring"test", unsafeAddr(test[0]), cint(test.len-1))

  return 1