Can you sign up just as a dog walker and do drop in visits?

Yes, you can choose to whichever service you want. Rover is available through the US and Canada. Whether there currently are any sitters in your area, I don't know. But you can do a search yourself.

There are lots of sitters and walkers in your area https://www.rover.com/dallas--ga--dog...type=city&middayavailability=true