Can I freeze stew made from leftover roast beef?
Yes, you can freeze and reheat the stew. Just keep in mind the total amount of time the meat is between 40°-140°F. If you make the stew and serve some and immediately chill and then freeze the leftovers then you should be fine. If you leave the stew out for several hours before freezing then it may be a little iffy. 

Also, you don't want to be putting warm items into your fridge and freezer, so a good course of action is to chill in an ice bath (in the sink or beer cooler works well) and then put into the fridge or freezer. Otherwise you're warming the temperature of your fridge/freezer and causing everything else stored in there to spoil prematurely. Obviously don't put the meat directly into ice water, but put it into, say a glass storage container or if it's cool enough (below 190°F) a freezer zip-lock bag is great.