Cool! Got it working, like to share it with you:
Code:
VBoxManage setextradata "my2CentOS" "VBoxInternal/Devices/e1000/0/NAT#0/Config/guestdb2/Protocol"    TCP
VBoxManage setextradata "my2CentOS" "VBoxInternal/Devices/e1000/0/NAT#0/Config/guestdb2/GuestPort" 50000
VBoxManage setextradata "my2CentOS" "VBoxInternal/Devices/e1000/0/NAT#0/Config/guestdb2/HostPort"  50100
db2 -v "catalog tcpip node                         vCent remote localhost                   server 50100" 	
db2 -v "catalog database sample as vCentDB at node vCent"
#
Now I can connect to the db2 running as a guest (virtualBox with NAT network).