Same SNMP item with different results

[pcraponi]

Hi,

I monitoring some devices using SNMPv2. Since is the same device, but with different models, the same OID can return 1 or more results for different model. Like this:
--
Customer 1:

snmpwalk -v 2c -c xxxxxxx 10.2.2.2 FORTINET-FORTIGATE-MIB::fgVpn

FORTINET-FORTIGATE-MIB::fgVpnSslStatsLoginUsers.1 = Counter32: 0
FORTINET-FORTIGATE-MIB::fgVpnSslStatsLoginUsers.2 = Counter32: 8
FORTINET-FORTIGATE-MIB::fgVpnSslStatsLoginUsers.3 = Counter32: 0
--

Customer 2:

snmpwalk -v 2c -c xxxxxxx 10.10.10.1 FORTINET-FORTIGATE-MIB::fgVpn

FORTINET-FORTIGATE-MIB::fgVpnSslStatsLoginUsers.1 = Counter32: 2
--
(The same OID, differente results)

I want an item that sum these lines even this result 1 or 10 lines. I know that I can create a Discovery rules using {#SNMPINDEX}, but how I can create just 1 graph with the sum of all items (index) ?

I'm using Zabbix 2.03

Best Regards,
Paulo Raponi