Use a combination of min and max delay constraints between the feedthrough ports. Example:
set_max_delay -from [get_ports din] -to [get_ports dout] 10 set_min_delay -from [get_ports din] -to [get_ports dout] 2
This is a simple way to constrain both minimum and maximum delays on the path. Any existing input and output delay constraints on the same ports are also used during the timing analysis. For this reason, this style is not very popular.
The max delay is usually optimized and reported against the Slow timing corner, while the min delay is in the Fast timing corner. It is best to run a few iterations on the feedthrough path delay constraints to validate that they are reasonable and can be met by the implementation tools, especially if the ports are placed far from one another.