Hi Kevin,
We've done it various ways in the past,
So you'd need DLP to send the incident info to SN, this could be done via the flex response,
then for updates made in the SN ticket you'd need to build a script/integration that would pass this information back to DLP.
If updates are made to the DLP incident after it has been raised you'd need another way to get that new/updated information over to SN.
Workflow could be used as a middle man to manage the dataflow, but it's not the only option.
Since you are already using SN as your ticketing system I wouldn't see a need for Symantec ServiceDesk as what purpose would it be filling? You'd have 3 tickets then, 1 in DLP, 1 in SD and 1 in SN.