Like most of Nokia's stuff, the applet in question is closed source, so, no, you can't modify it to add more timeouts.

What you can do, however, is change the setting that would be saved from that applet yourself:
gconftool -s -t int /system/osso/connectivity/network_type/search_interval -TIMEINSECONDS-

I haven't tried adding higher numbers myself, admittedly, but I suspect whatever is entered there is given straight to a g_timeout_add_seconds () in the closed source ICD2.