Hi,
The easiest is to hide the original button by adding this to your custom css:
#newItemWrap_vLookupTasks {
display:none;
}
Then add a button to your button-row like this:
<input type="button" onclick="jQuery('#newItemWrap_vLookupTasks > span').trigger('click')" value="Add new item">
Replace vLookupTasks with the internal name of your field in both snippets above.
Alexander