I've managed to find a solution, and I'm sharing it here in case others encounter a similar issue.
For the creation of a backward trajectory in RR, a feature called Actor Orientation must be enabled.
Actor Orientation symbol :
To do the enabling, a specific configuration should be modified while launching RR.
Apart from directly launching, The typical method to launch RoadRunner is by executing the command : AppRoadRunner.exe
To execute RoadRunner with Actor Orientation enabled, use the command : AppRoadRunner.exe --MW_RR_FEATURE_ActorOrientation=ON
If properly enabled, the Actor Orientation icon should be visible at the top, alongside other simulation icons. :
By selecting Actor Orientation and then choosing a vehicle, you can adjust the vehicle's direction in relation to the center waypoint.
This helps us align the vehicle the way we need.
So, if we set Actor Orientation - Heading Delta to 180, the waypoint will now face the backward direction of the car. This facilitates achieving reverse driving.