Depends on how you want it to look, I guess. You could use green stuff to sculpt it, that would be rather intensive and require a bunch of green stuff. The idea I had for my Assault Marines this:
1) put the pins in place on the base where you need them to be (using the fig to determine where they need to be)
2) get a can of this: http://www.lowes.com/lowes/lkn?action=productDetail&productId=15634-236-230612&lpage=none
and SLOWLY spray the foam onto the pins, bringing the sprayer up as you spray it, and it moves up the pin basically.
This stuff is sculptable once it dries, using a serrated knife, but it is MESSY, so be careful what you wear and where you use it, as it does NOT like coming off of clothes/furniture.
3) then, attach the vents of the jump pack to the tops of the pins so it looks like the foam is coming out of them. You could even paint the foam, probably, to look more like smoke...
This could very well work, I haven't tried it yet, but eventually, as I get more Assault Marines, I'm going to try it.
A tiny portion of humanity turned its back on mankind in the waning days of the last Golden Age. Over one and a half centuries later, the PHR has emerged from the shadows as an unrecognisable civilisation, its people irrevocably changed. They are no longer simple human beings, they are post-humans - cyborgs.