I don't think you have access to the question form inputs directly enough to indicate a tab order. If you really think it's worth it, you could create the question so that the taborder gets set as you wish. Below is a question code example that uses floating divs to fool the question layout (tab) order.
<p>Perform the following 2-digit multiplication</p>
<div style="clear:both;">
<div style="float:right;width:4em;text-align:right;">3</div>
<div style="float:right;width:4em;text-align:right;">2</div>
</div>
<div style="clear:both;">
<div style="float:right;width:4em;text-align:right;">4</div>
<div style="float:right;width:4em;text-align:right;">3</div>
<div style="float:right;width:4em;text-align:right;">X</div>
</div>
<div style="clear:both;">
<div style="float:right;border-top:solid thin black;">{:SA:=2}</div>
<div style="float:right;border-top:solid thin black;">{:SA:=8}</div>
<div style="float:right;border-top:solid thin black;">{:SA:=7}</div>