SymbolicControlSystems.jl contains a number of functions for converting to and from sympy, but the support for Symbolics.jl is not that well developed. However, it does look like the following works
julia> using Symbolics
julia> @variables s;
julia> (s+1) / (s+2);
julia> using SymbolicControlSystems
julia> G = (s+1) / (s+2)
(1 + s) / (2 + s)
julia> tf(G)
TransferFunction{Continuous, ControlSystemsBase.SisoRational{Float64}}
1.0s + 1.0
----------
1.0s + 2.0
Continuous-time transfer function model