I was checking web site of the FoundationDB. It says that it was developed on new programming language Flow. In websites it says :
Flow is a new programming language design -- in fact an entirely new programming language paradigm -- that solves the multicore dilemma through ubiquitous, guaranteed-safe implicit parallelization.
The code
itself eventually is being compiled to C++.
What I am thinking about is that would it be possible to have C++ code inside Flow and would it be possible to reuse already existing C++ code/libs ?
There is a bit of confusion here. FoundationDB is built using a proprietary language called Flow, whereas your link to Flow (flowlang.net) is a different programming language. The naming is unfortunate, but understandable as FoundationDB's Flow (which I'll call FDBFlow) was started prior to Flow(lang), but not publicly disclosed.
In brief: