To add the spell check button, do this to your modifysignature templete,
Find:
Code:
<input type="submit" class="button" value="$vbphrase[preview_signature]" name="preview" accesskey="r" tabindex="1" />
Below this add:
Code:
<input type="button" class="button" value="Spell Check" onClick="spellCheck('qrform', '$editorid', '2');" />
That should have your spell check buttons everywhere except for the Calendar, if anyone wants this and don't know how, let me know.