Shourtcut for slow down the animation, without changing the frame duration

Hey everyone im workin on 120 frame and 30 fps. for example, I am preparing a 120-frame character animation. but i am creating an animation of a light on the character in a new layer. let’s say this light animation consists of 30 frames, but I need 4 of each frame to distribute it into 120 frames.(i want the light to animate slowly.) so I need to duplicate 30 frames one by one.
is there a shortcut I can do without changing the frame duration?

Not that I’m aware of. Aseprite lets you Duplicate multiple cells at once (optionally Linking the copies), but it duplicates the whole group in order each time, so you get 1 2 3 4 1 2 3 4 instead of 1 1 2 2 3 3 4 4. Would be nice to see an option to duplicate each cell in place, since that’s useful about as often as duplicating cels to repeat the whole animation.

What you need would have to be done via script, either one that fills all the empty cells in a layer with empty copies of the previous non-empty cell (if your 30-frame animation starts off spaced out in the 120-frame document), or one that duplicates all the frames N times and makes the cells linked (if your animation starts off as a separate 30-frame document you plan to paste into the 120-frame document).