I finally got the solution. I posted into luci github forum, and they told me that "luci-app-statistics" doesnt support collectd-mod-snmp. And they invited me to see the data model of any other collectd-mod-xxx in order to create my own collectd-mod-snmp data model so luci-app-statistics should support it.
Since snmp is so wide, its impossible to get a closed data model, but i think i got a workarround.
firstly, its quite important install collectd-mod-syslog in order to read all log entries so you can see which modules are needed in order to get the graph working.
Later, you can use all collectd-mod-xxx which are already supported, in order to simulate same folder structure. For example, i queried via snmp the uptime of the device. As i already had installed collectd-mod-uptime, there was really easy make it works.
this is the folder content of a collectd-mod-uptime device:
This is the folder content of the snmp device:
The final solution is to create a symlink this way:
$Collectd_Folder/snmp_device/uptime/uptime.rrd -> $Collectd_Folder/snmp_device/smmp/uptime.rrd
Now, luci-app-statistics, draw the graph as this snmp device, really was created with the collectd-mod-uptime instead.
So, be carefull to query via snmp same values that collectd-mod-xxxxxx provides to luci-app-statistics. (thats why you must install collectd-mod-syslog, so you can know which files collectd-mod-xxxx are expecting, or you can also see the proper datamodel to see which files are generated)