Network Sonar builds a database of all devices on a network. You can import the list of IP addresses into SNMP Dictionary Force Attack directly from the Sonar database. See Network Sonar.
You can also use Ping Sweep to build a list of IP addresses. After scanning an IP address range with Ping Sweep, export the IP addresses to a comma-delimited file. You can then import the file into SNMP Dictionary Attack. See Deprecated Ping Sweep.