Can I turn a can of red kidney beans into red bean paste for Japanese Style Red Bean Paste Buns?
Welcome. According to the excerpt below from  this page , kidney beans are an acceptable substitute.


  azuki bean = adzuki bean = Tiensin red bean = aduki bean = asuki bean = field pea = red Oriental bean = feijao bean = red chori    Equivalents: 1 cup dried yields 3 cups cooked beans  Pronunciation:  a-ZOO-kee   Notes:   The Japanese use these small red beans to make sweet red bean paste, but they're also good in rice dishes or salads.  Azuki beans are sweet and relatively easy to digest, so they won't make you as gassy as other beans.  They also don't take as long to cook.   Substitutes:   black azuki beans OR red kidney beans OR Tolosana beans


That said, while you may be able to substitute them you may need to make adjustments to end up with a similar bean paste. For example you may need to add more sugar as adzuki beans have a natural slightly sweet flavor.

Also, even though you have canned beans, they may require a bit of cooking to get them to the right softness. (Adzuki beans tend to have a very creamy texture.) Note that dark red kidney beans tend to be more firm than light red.

All said and done, you can probably make adjustments and end up with a slightly different result. But as for me, I would get the adzuki beans.