There is not. The IP address is extracted from the IP packet header, and Splunk does not interfere or generate at that protocol layer. This is why Splunk (like other syslog agents) can prepend the data and the IP address before forwarding. The setting
no_appending_timestamp = false should be set on the Splunk UDP input to make Splunk do this.
If you must have spoofing, just have syslogNG receive the data instead of Splunk. SyslogNG can break IP via source spoofing, as well as write to files that Splunk can index.
We have customers using rsyslog as well to write incoming syslog traffic to directories by host and splunking it just fine. They have written a lot of it to different directories so that they could have multiple splunk forwarders consuming the data. They broke out the busiest firewall traffic specifically so that it could handle the amount of data being written. They are collecting over 1.2TB per day on rsyslog.