Not sure if I understand what you are asking, but it sounds unsafe.
DOT 4L liquid cylinders are designed with safety features in place that should not be circumvented. They are not designed to be high pressure containers, and will periodically vent through their pressure relief valves - which keeps them from becoming deadly bombs. There are a number of cases where university labs have plugged the pressure relief devices, and the cylinder has exploded, causing massive damage to property.
Besides which, making an alteration of any kind to a DOT 4L cylinder is a citation of both your agreement with the compressed gas supplier and with certain DOT regulations.
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