Possible but not guaranteed.
It would require "tape monitoring" jacks for at least five of the channels in your 5.1 system. Assuming the equalizer itself has at least five channels.
The normal signal path for audio is, source device -- receiver input jacks -- selector -- optical to analog converter or HDMI to analog converter if applicable -- preamp -- tone controls -- power amp -- receiver output terminals -- speakers.
You need to tap out the audio after "preamp", run the audio through your equalizer, and re-introduce the audio back into the receiver to go through the power amp stage. Tape monitoring jacks provide just this capability.
Your old receiver must have had the same tape output jacks, possibly called by a different name, to permit adding on the equalizer.
Video hints:
http://members.aol.com/ajaynejr/video.htm
You could run the DVD player audio through the equalizer first and then to the receiver with no tape outputs. Here only one source device (one DVD player) would be served by the equalizer (unless you added an audio switch box as well). you only get right front and left front audio channels (red and white cables). Also players with only optical 5.1 and red and white audio will give you only get right front and left front audio channels.
