This is example of editing wiki page , see below

<tr>
<td>open</td>
<td>http://wiki.test.com/index.php/asdf</td>
<td></td>
</tr>
<tr>
<td>waitForElementPresent</td>
<td>link=Edit</td>
<td></td>
</tr>
<tr>
<td>clickAndWait</td>
<td>link=Edit</td>
<td></td>
</tr>
<tr>
<td>storeText</td>
<td>wpTextbox1</td>
<td>wik1</td>
</tr>
<tr>
<td>type</td>
<td>wpTextbox1</td>
<td>${wik1} ${wik2}</td>
</tr>
<tr>
<td>clickAndWait</td>
<td>wpSave</td>
<td></td>
</tr>

wik1 is the old content wik2 is the new text that will be added to the text area (text box)

BR,

Ramadan

Did you try it ? Is it working ?