Can I use cow feet in pho broth My instacart shopper has accidentaly given a pack of cow feet, in addition to the soup bones I ordered.  

Can I boil the feet alongside the bones while making pho broth?  

Cow's feet are primarily used for making stock or stews/soups due to their collagen content, which lends body and umami to the end product.

This makes it sound like a great idea, and it would act just like tendon in the soup.  Is there any reason not to do this?  I've never cooked pho or cow's feet before.
You can. Cows' feet will add gelatin without adding much meaty taste, which may not be ideal for pho both (depending on the other ingredients). In my experience they take longer to render than pork gelatin. They can also add off flavors if not treated properly (similar to tendon); blanch them in a couple of changes of water before you use them.