I am afraid that there is no easy way to input a user defined hydrograph when doing a suite of runs. The user defined hydrograph must be inputted into the storm file and currently there is no way for RORB to automatically put the user defined hydrograph into the storm file when RORB generates them.
What you would need to do is to manually add in the hydrograph to each storm file generated (details on how to do this are in the RORB manual, Table 5-2 Item 9), add in the inflow to the catchment file (after generating the storm files) and then re-run each simulation individually with the altered storm and catchment files. This may not be practical depending on the number of runs you are doing.
To help with running the individual simulations, you can try to batch up the individual runs using the command line version of RORB. The instructions on how to use the command line version should be included in your RORB download.