I am sure that I will be corrected if I am wrong, however I understand the following to be true.
HSE / DOT stamping can only be carried out at manufacture.
As far as I am aware the difference between the standards is minimal.
Neither the US or Europe are going to allow a bottle to be dangerous in either continent. It is all down to politics.
However paintball sites, in the UK, are only insured for HSE certified bottles. If you are using a DOT bottle, you are not insured and are therefore, making the site operator break the law by allowing you to play.
I have played several tournaments in the Millenium series, and have allways had my bottle checked prior to the first fill, until Campaign this year. Nobody checked any bottle for either DOT/HSE 4500/3000, or date of test. That was a joke.
An example for you:-
One of our least experienced players went to the fill station with his 3000 psi fibre wrapped bottle.
He managed to get 4500 psi in it as he went to the wrong fill rig. The fill station was not identified, and his bottle not marked with maximum pressure as at Tolouse. (Different coloured cable ties etc.)
If you require information as to the legality of your air system, contact Steve Bull at the UKPSF
Yours as allways P'D Off