Jegs makes a hard line to AN adapter fitting. I bought this to adapt to the ZZP rails because the ZZP adapter was leaking.
JEGS 110551 - JEGS Hard-Line AN Adapter Fittings - JEGS
Swagelok also makes a hard line adapter, but I've never used this:
SS-600-6-6AN - SS Swagelok Tube Fitting, Union, 3/8 in. Tube OD x 3/8 in. AN Tube Flare