Hello Randy, Yes you can add an a PERC6/i to your T7400 as the PERC card was optional addon for the machine. please see the link below & see the tech specs where it list the PERC 6/I as optional.  [URL]  Please let us know if you have any other questions.
Will the  PERC 6/I RAID Controller w/256MB cache and battery work on a Dell Precision T7400?Randy
 PERC 6/I RAID Controller w/256MB cache and battery