No, it is not an option in iWeb.
If you want to do this, then you will have to write the html link code yourself and enter it into an html snippet, or you really need to use a third party application or widget to allow you to do this.
Just as you have html links, such as the:
<a href= you can try <a href="sms: (insert your mobile number here)">Text Me</a>
This may or may not work, but most likely you will need to hunt around for a third party application where you can copy and past the code into an html snippet in iWeb to get it to work.
Remember nowadays, that nothing is that easy to accomplish in iWeb and iWeb is very old software and you probably need to look at re-building your site in more current software - just look at this forum for the many alternatives available.
It is not really a good idea now to use iWeb to build a site from scratch - you need to be using current and up-to-date software to do this.