Hello. Importing images as layers is not possible in v0.7, but will be in the next version. Keep in mind that imported layers will be cropped to the project's size, so if you were to import a layer of a bigger size than the current one, it would get cropped.
Right now what you could do is make selections of the entire canvas, copy the imported frames, and paste them as new layers. Copy & paste works only with the rectangle select tool currently. Then, you can remove the extra unneeded frames.