I don't see why this would be a problem. This would probably be better for juvenile frogs thank adults since brine shrimp are quite small but for enrichment (the actual hunting of prey would be stimulating) I see no problem with this.

I would just add a scoop of brine and see how the frogs react and give them a few hours and see what happens. Xenopus are big eaters and I imagine if you added a ton at once they would simple 'go nuts', they are very prone to overeating.