There are two options. Ayman's approach is the most simple, but I would add one extra note to it. You should really cache jQuery selections, there is no reason to call $("#input-field-id") twice:
// Define appendVal by extending JQuery
$.fn.appendVal = function( TextToAppend ) {
return $(this).val(
$(this).val() + TextToAppend
);
};
//_____________________________________________
// And that's how to use it:
$('#SomeID')
.appendVal( 'This text was just added' )
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<form>
<textarea
id = "SomeID"
value = "ValueText"
type = "text"
>Current NodeText
</textarea>
</form>