Should be fairly easy, you can define custom functions in the google sheets app and just run it - here's a quick example
Code
function addRows() {
var numToAdd = 10;
var minDelay = 1; // in seconds
var maxDelay = 3; // in seconds
for (i = 0; i < numToAdd; i++) {
var currDelay = Math.floor((Math.random() * maxDelay) + minDelay);
Utilities.sleep(currDelay*1000)
var sheet = SpreadsheetApp.getActiveSheet();
sheet.appendRow(['foo']);
}
}
You might have to modifiy it a little bit to pick the starting spot ( this starts at the next avail row )