As suggested above, you can try:
(1) The injection file. For that you’ll need to specify parameters such as
(( x y z u v w diameter temperature parcel-mass mass n-in-parcel time flow-time) name)
in a raw ascii file. You can control the injection by start-time and stop-time.
(2) Alternatively, if during the pre-processing you prepare a mesh that distinguishes a fluid region where the parcels are to be injected. You can later use an “artificial” surface injection selecting the proper int_fluid region. Then, the parcels will “sprout” from the face centres of such region.