Unfortunately, the answer is no, you cannot.
From what I understand of the fuction
r.watershed, the subbasins cannot be merged, which is what I guess you want to happen.
I understand your question and your interest in merging small catchnment areas downstream exterior basins, but the way they are defined in this fuction depends on the exterior basins and the network that can be derivated from it. It is not possible to have a catchement area with more than one exterior basin, because you cannot have a river with two starting points.
I guess you are interested in building a topology that allows a subcacthment area to have more than one exterior basin or keep them growing downstream. This is indeed interesting for hydrology purposes. I know the SMHI has a tool related with this task (WHIST - World Hydrological Input Set-up Tool), but it has very little documentation. There is more documentation about their subbasing generation precedure in their hydrological model Hype.