Not sure if this will work, but try to put each PS frame into it's own layer in Fresco - then, apply animation on the layer (frame) you want to copy. Go to the layer timeline, tap on the thumbnail for that frame and choose copy frame.
Then, start a new, fresh layer and apply animation, so you can see the time line for that layer, tap on an empty frame and choose paste frame.
So, this new layer would be your new animation timeline and you would repeat the steps to get the other frames copied and pasted into that new layer timeline. I think if you select the last frame thumbnail, then tap paste frame, it pastes it in a new frame right after the one you selected. Hope that works!