Is it possible to create a texturepack from a renderTarget?

pie

I usually do this:

local pack = TexturePack.new("pack.txt", "pack.png")

But this time I need to pass pack.png through a shader before, which gets me a renderTarget object with my shaded texture.
is there any chance to switch pack.png with the rendered texture?
My goal would be to use pack:getTextureRegion() using regions defined in packs.txt, while using renderTarget element as source image.

Any help is appreciated, thank you