Thanks for the replies, but the problem is different.
The SSID has only wep with a pre-shared key and the pre-shared key is already in the hexadecimal format.
New PC clients can authenticate and associate to both the old network and the new one (the new one is the one made by the Aruba APs) without any change in the configuration but the existing legacy clients (that are integrated in some serial appliances with no user interface) authenticate and associate only to the SSID propagated by the old APs.
I've made a capture of the wifi communication using an AP as a wireless sniffer and I see that in 802.11 authentication frames the client set the Authentication Algorithm flag is set to 1 (Shared-key authentication) and the Aruba replies with a status code 0d00 that is: the AP does not support the required authentication algorithm.
Searching for troubleshooting 've found this document:
www.connect802.com/download/aruba/Aruba_Client_Troubleshooting.pdf
where at page 6 it says that Aruba indeed does not support shared-key authentication.
I know about the weakness of the system but I don't think the customer will be willing to change the configuration of the legacy clients to something different (which is actually no easy task).. so I'm looking for a workaround.
Any ideas?