With lines.txt:

  printf '(%s)\n' "one two" "three four five"
  echo "How you doin'?"

Running:

  while read -r line; do
    eval command=($line)
    command+=("arg with spaces" "other arg")
    "${command[@]}"
  done < lines.txt

Yields

  (one two)
  (three four five)
  (arg with spaces)
  (other arg)
  How you doin'? arg with spaces other arg