I had no clue what he was asking me. I felt so sorry but I asked him to say that again and again, for several times, and I finally understood what he said was "Do you want it warmed up?". I actually realized when he pointed the microwave oven there. The thing is, I never knew that the cinnamon roll was to be warmed up before eating. Since I have learnt this, I can catch up the words even if it still sounds “Juwamiwamidap?” to me.
Yes, there are food items that can be warmed up in convenience stores.
When buying food items such as Bento (lunch boxes) in a convenience store, you might be asked at the check-out counter if you want them warmed up with the phrase "Kochira Atatame masu ka? こちら温めますか", means “Would you like it(this) warmed up?". Also, some people say "“Kochira Attame Masu ka?" in casual way as “Atatame Masuka” is hard to pronounce. It recalls me of my English teacher complaining about the Japanese phrase “Atatakakattakara” (= As it was warm) because of too many ta and ka.
In case the store staff doesn’t ask you and if you want your food warmed up, just say:”Atatamete kudasai 温めてください” or "Attamete kudasai", equivalent to “Can you please warm it up?”
Onigiri rice balls are to be warmed up or not to be?
Though a data says nearly 70% of people don’t ask for warming over Onigiri in convenience stores, it actually varies depending on regions. In Hokaido, Okinaka or those specific regions it is taken for granted that Onigiri is to be warmed up. In Tokyo the store staffs rarely ask customers if they want their Onigiri warmed up as most customers don't care for.