Yes you can. See below, where I have grabbed the textbox value and concatenates it with an additional value.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title>Test</title>
<script type="text/javascript">
function addText1()
{
document.test.text1.value = document.test.text1.value + " THIS IS EXTRA! "
}
</script>
</head>
<body>
<form name="test">
<textarea name="text1"></textarea>
<input type="button" value="Add Text" onClick="addText1();">
</form>
</body>
</html>