You can try using this function
function insertRow() {
var spreadsheet = SpreadsheetApp.getActive();
spreadsheet.getRange('A1').activate();
spreadsheet.getCurrentCell().getNextDataCell(SpreadsheetApp.Direction.DOWN).activate();
spreadsheet.getCurrentCell().getNextDataCell(SpreadsheetApp.Direction.DOWN).activate();
spreadsheet.getActiveSheet().insertRowsAfter(spreadsheet.getActiveRange().getLastRow(), 1);
spreadsheet.getActiveRange().offset(spreadsheet.getActiveRange().getNumRows(), 0, 1, spreadsheet.getActiveRange().getNumColumns()).activate();
};
It takes you to the last cell on column A (no matter if it has any data or not, as long as it doesn't have any gaps/empty rows on the way down. If it does, just add extra copies of the getNextDataCell(SpreadsheetApp.Direction.DOWN) code line until it gets you there), and then adds a new row after, so you can call it before running your Importrange().
Also, if you want to add more than one row at a time, just change the last argument in
spreadsheet.getActiveSheet().insertRowsAfter(spreadsheet.getActiveRange().getLastRow(), 1);