I do this sometimes when I'm not doing a feeding with vitamin dustings. I just take the lid off my FF culture and place the whole thing in the tank for an hour or two. I keep that culture assigned to that tank so I don't cross contaminate. But I have found that the flies do not wander far away from their food source, and the hang out at the top of the culture near the rim. The frogs see the activity and go to town on the flies. Some flies wander and some flies crawl out, but thw majority hang out near the culture.
A word of caution with this technique especially if you use excelsior. Be sure there are no frogs inside the ff culture when you take it out!