Just to recap, Divx, MPEGs & Flash type files are all meant for delivery purposes (people to view) not edit. However, can you edit them, of course, just not frame accurate or PQ suffers. Can you convert them, of course, same issues. With proper plugins and codec (encoders and decoders) installed, you should be able to do so.
so, Numsen, why are you not able to render them out? My suspicion, if you do have the proper encoders installed, is that you are going into the wrong place to render (in PPro CS3, for example, you have render via Media Encoder, render movie… so confusing). Best wish.
Perry