Is there a way to add a Captcha to Ubercart?
I am assuming you are talking about the add to cart form. Please share on what page or place you need captcha?

If you cannot find a module, then you need to do exactly what the captcha module does,


Write hook_fomr_FORM_ID_alter for the form for which you need to add the captcha.
Look for articles online explaining how to add captcha programmatically on a form in Drupal, ex: How do I add reCaptcha to a custom form?, https://www.drupal.org/node/743056.
Enable your custom module containing this code, and the captcha module. 


You should have your captcha on ubercart's add to cart or any other form with a submit button.