Hi
The AP can have the same SSID for roaming, but technically, its not called Roaming.
Say you have an office with 2 APs, both configured with the same SSID,
Now a client is connected to one of these 2, moved far from the AP and the signal became weak, then the CLIENT will disconnect from that AP and it will find the other AP with same SSID and strong signal so it will associate with it.
Again, the ProCurve 420 is a wireless access point only and can;t work as a repeater or in bridge mode.
Good Luck !!!