Skeletal mesh animation - start from current bone transform

Hi. I’m learning how to work with animations.
I have basic skeletal mesh with two bones. Each of them has the primitive attached static mesh - cube:
изображение
I would like to rotate one cube around red axis (which connects red points - bones). I have animation which rotates cube from 0 to 30 degrees, it works. But when I call this animation again - it resets cube angle to 0 and rotates from 0 to 30.
Is there any way to continue rotation? First animation call - from 0 to 30, second call - from 30 to 60… ?
My weapon has drum magazine, which has two has 3 animations: rotate 10, 20 and 30 degrees. For now I use timelines in C++ to rotate parts of weapon static meshes. But I want to implement weapon as sekeletal mesh and use animations from Blender.