Green stuff

Or milliput - whatever your preferred squashy stuff is
Do you want them to be newly tainted, fresh traitors, with just the beginnings of mutation...?
Or seasoned Chaos fanatics, serving their dark lords for millenia in the eye of terror...?
For the back packs you could simply carefully remove the old ones and use regular plastic chaos backpacks, or you could do a number of things to show them as Chaos...
Chaos, for some reason, tend to have the \"exhausty-bits\" of their backpacks extended, so you could simply carefully remove the normal exhausts - being careful to keep their shape.
Then you could drill into the backpack and exhaust, and pin a lengthened pin into place. Then simply disguise the joins with green stuff.
If you wanted, you could even exchange the regular exhausts for skulls, or something...
Chaos has a range of options for converting, but as already stated - you need to make some choices first.