Unless you use frames and rewrite the complete form, only Internet Explorer 4+ can do what you want:
Amos Bannister writes:
The solution to this question is not entirely correct. Two possible solutions come immediately to mind, both of which I have used successfully in the past, and both of which work fine with both Internet Explorer and Netscape. A summary of these solutions is below - I can elaborate if you wish.
Solution 1: Create another FORM containing a TEXT INPUT control. Wrap the form in a <DIV> and set the visibility to invisible. When you want to "create" the text box, just make the DIV visible and position it where-ever you need.
Solution 2: It is possible on both Netscape and Internet Explorer to dynamically create layers (actually DIVs, but they're the same thing kind of). So a more advanced solution would be to create a new layer and write a new form definition to the layer. I have created a set of DHTML widgets based on Dan Steinman's DynAPI which do this quite nicely.