With enough money, you could probably do it, but it's not worth it. You might be able to use some of the boxes and re-weld some parts, but you may as well put that effort in doing one properly for YOUR car, instead of adapting another one. Also, the labour involved in redoing the E60 exhaust may be more than doing one from scratch on your car. What you'll save on parts, you'll spend on labour, but end up with an inefficient system.
Lastly, decat (or specifically de-DPF) means little if it's still the OEM box, as you don't have optimum exhaust gas flow by just gutting out the insides - you'll get some gain, but not the full benefit.
Hence, rather do a proper install from scratch on your car and have one custom designed for your requirements (power and sound wise).