Is it ok to cook beef from frozen in a slow cooker?
It's a good question since you need to be careful with keeping food in the "danger zone" (40°F-140°) for more than two hours (or more than one hour at 90°). In a 200°F oven, for example, a large chunk of frozen meat will take a long time to thaw all the way to the center. 

Cooking from frozen is generally accepted as safe, and the USDA has an article dedicated to frozen meat. They say it is safe to cook from frozen and will take 50% longer to cook, but it's more complicated than this. 

When you're cooking with direct heat transfer (such as sous vide or in a stew), the heat transfer is going to be much faster — especially with sous vide where convection is used. A steak in sous vide will probably thaw within 15 minutes depending on the thickness. In a slow cooker it depends on how much liquid surrounds the steak and what temperature you're using. I would probably add 30 minutes of cook time. 

Also keep in mind that with a steak the bacteria is going to be on the outside, so cooking it first (as in searing) will kill most of that bacteria, making a slower cook for the inside safer. This is also a reason not to poke holes in your steak when cooking rare.

Cooks Illustrated also has an interesting article about cooking frozen steak.