Add additional tag within all span tags using c#

Hello.
I have below HTML string.

<span class="headinglabelonimages" id="lbl133030-1" style="text-align:left;width:30px;height:10px;font-size:22px;font-weight:bold;color:#000000;"><fseparator><id_133030_1><span>Question Title</span></id_133030_1></fseparator></span>

In c#, I want to add <p> </p> tags within all <span> tags. So that, out put looks like,

<span class="headinglabelonimages" id="lbl133030-1" style="text-align:left;width:30px;height:10px;font-size:22px;font-weight:bold;color:#000000;"><p><fseparator><id_133030_1><span><p>Question Title</p></span></id_133030_1></fseparator></p></span>