Harold,
First i would like to ask -
1. What type of information in the header you would like to based your steering on?
2. What is the reason to go for DPDK? do you need higher performance?
What you're probably looking for in DPDK is the Flow Director feature, which will be supported in the next MLNX_DPDK release (End of Feb), however, only for ConnectX-4 and not ConnectX-3. In general, i would advise to move to ConnectX-4 if possible (for sure will be much easier than hacking the code ).
In DPDK, you can also change the RSS RETA table (i.e manually configure the hash result -> queue mapping)
Same thing (Steering, not change RETA table) can be accomplished with ethtool if you are using the standard driver.
Note that not all HW support all fields, hence my first two questions.
Hope this helps and let me know if you need more info.