If you want to cold crash the standard vessel and apply 2 PSI to the flex, this can be beneficial to avoid suck back of oxygen or sanitizer that can occur.
If you're looking to do a low pressure transfer, we do not currently have a bolt on solution but one of our engineers quickly assembled a part that would work. It is an airlock bung, gas post and a ball lock gas fitting (see attached). This can attach to your CO2 source. However be extremely careful to not overpressurized your FLEX.