You would need a licence from Motorola in order to get the Waris software to flash the radios (in other words you need access to MOL- Motorola Online).
Im not sure if Rob (Admin on the forum) can sell you the flash software... I know he can sort you out with CPS related business, but not sure where you stand with regards to purchasing the flash software! Might be worth dropping Rob a PM (or use the 'Contact Admin Team' tab at the top of the page).
Sadly in todays age we are no longer allowed to 'share' software. Before I joined this forum there was a download section with everything you would need. Motorola caught onto this and, from what I have read and been told, they were in the process of taking legal action against the forum owner. Hence the reason/rules that
you cannot canvas for software in an open forum.
Converting a GP340 into a GP640 is not 'common practice', so there is no specific firmware to conduct such a process. The method that Andreas has explained is the way he suggests-and it works, but from an official Motorola perspective they would not want you to be able to do such a thing- and would most probably tell you it is impossible... They would rather you go out and spend more money on a 'proper' GP640!

Bearing in mind you will also need the Unofficial Homemade EU Waris Lab Flashkit- which I have no idea where you can get hold of that im afraid

Have a chat with Rob and see what he suggests!
BW