There is a field of regenerative medicine to treat strokes and TBI, which involves activation of endogenous neural stem cells or grafting various stem cell types into a brain, developing them to neurons.

I see in such papers, that those newly formed neurons apart from projecting their axons to appropriate targets, also obtain inputs from corresponding host neurons. (e.g Article Transplanted embryonic neurons integrate into adult neocorti...

Article Donor-derived vasculature is required to support neocortical...

So the question is, how is that possible? I mean, there's either a synaptic connection between 2 neurons or a volume transmission mechanism.

It is understandable concerning the latter one but is that possible, that the fully mature host neurons make new axons (i.e. "projections") to newly introduced neurons?

Maybe those new neurons can grow huge dendritic trees, so they can reach residential axons...