I like to add a bit of bird seed to my hammy mixes.
Especially as the are on Burgess SupaHamster, and every bag seems to be different. My current bag of Dwarf seems to be nothing but peas, pellets and millet! There's hardly any sunflower, flakes or mealworms in it. :/ Not very impressed.
I give them a small amount (less than half a teaspoon) of Walter Harrisons Premier Wildbird mix, but I've used Foreign Finch or budgie seed before.
That mix you're mentioned looks to be fine. Don't give them much though as you don't want to upset the nutritional balance of their daily food.