I have recently purchased something from another member here on DP. However the OP of that thread was NOT the seller. How do I go about adding feedback to the actual seller who also participated in that particular thread, but didn't create it. Hope you understand my question
You cant. It is in the rules. You will have to refer to a deal thread that either you or that person started.
As JoyGoRound said. I'd just leave it. Things get messy if you start creating new threads just to leave feedback. iTrader is completely optional, so the seller would be wrong to make an issue of it.