I know that rabbits in nature have access to grains only for a few weeks in autumn. How can I use a purchased grain mix for them?
If you have grain food for rabbits or other animals (birds) and you have stopped feeding it, you could let it sprout.
Sprouts are a very healthy food for rabbits. Especially in winter season when fresh greens are limited, sprouts are important as a source of minerals and vitamins.
As sprouts, a lot of grains are well tolerated that in dry stage should not be fed.
It is very easy. All you need is a bowl and a colander.
Pour some grains and seeds in the colander. Clean them with warm water, then put the colander in the bowl and fill the bowl with warm water until the grain is all in/under water. Leave this for a few hours, but not more than overnight.
Drain the water and clean the grains in the colander again. Then put the colander back in the bowl.
Drain, clean, and refill the colander every morning and evening with warm water until the sprouts are out and big enough for your taste. Then you can give them to your rabbits and fill the colander again.
Remember that rabbits are smart: they try a small amount of new food and wait for one or two days to see if the new food will harm them. So you have to be a little patient to see if your rabbits like the new food.
(I will enlarge this answer with a list of suitable grains and seeds)