makeRangeAbsolute.gs

function makeRangeAbsoluteReference() {
  // This grabs the currently selected cells,
  // applying _makeAbsolute() to each
  var spreadsheet = SpreadsheetApp.getActive();
  var range = spreadsheet.getActiveRange();
  var numRows = range.getNumRows();
  var numCols = range.getNumColumns();
  for (var i = 1; i <= numRows; i++) {
    for (var j = 1; j <= numCols; j++) {
      var currentCell = range.getCell(i,j);
      _makeAbsolute(currentCell);
    }
  }
};

function _makeAbsolute(cell) {
  // This function accepts a reference to a cell which in turn
  // references another cell relatively. The cell reference is updated
  // to an absolute reference, ex. "C4" => "$C$4".
  // Currently only works for direct cell references with no manipulation
  // (won't work for calculated values such as "C4 + F2")
  var formula = cell.getFormula();
  var formArr = formula.match(/([A-Z]+)(\d+)/);
  formArr.shift() // get rid of entire match, pieces remain
  var absForm = formArr.map(n => `$${n}`).join('');
  cell.clearContent();
  cell.setFormula(absForm);
}