You can easily do that with javascript.
Copy the file below and test it.
...........................................................
<html>
<head>
<title>Dynamic Combos</title></head><body>
<!--
script provided freely by
-->
<h3>Select Author</h3>
<form action="
method="get">
<select name="myChoice" onchange="if(this.options[this.selectedIndex].text=='Other'){var c=prompt('Enter your Choice','');if(c!=''){this.add(new Option(c));this.selectedIndex=this.length-1;}}">
<option>Mark Twain</option>
<option selected>Micheal Crichton</option>
<option>Stephen King</option>
<option>Tanwani Anyangwe</option>
<option>Ian Fleming</option>
<option>Other</option>
</select>
<input type="submit" value="Go" />
</form>
</body>
</html>
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