Thank you
For now I used the interpolation and the injection-file, although I needed to decrease the number of parcels (from 16M to 8M , Fluent was leading to CORTEX FATAL ERRORS at the very beginning).
Also, for other users trying to do the same: I found convenient to reset the flow time to 0 s (rpsetvar 'flow-time 0) and to manually set columns 12 & 13 of the injection-file to 0.0000e+00.