Such works may be considered as copyright infringements under US law. The US does not apply the "rule of the shorter term", and so the PD status of the parent work in some other country,. including the source country, is not relevant. There are, however, other ways for a work to have lost copyright protection under US law, and one of them might apply in a particular case. Also, whether a work is legally a derivative work of another is a fact-based legal question, and the answer is not always as obvious as one might think.
The copyright holder of the parent work could file a copyright infringement suit, seeking damages and other relief in a US Federal court.
Note that one of the rights that make up copyright is the right to make or authorize the making of derivative works. This ism spelled out in 17 USC 106 item (2) "to prepare derivative works based upon the copyrighted work;"
I am not sure of the rules for customs seizure of allegedly infringing works. I thin k that only works already adjudged to be infringing are seized, pursuant to a court order, but I am not sure of that. II do not see how the customs service would determine the disputable issues mentioned above. For the matter of that, any such work could be authorized by permission from the copyright holder, and I don't see how customs would know if there had been such permission granted.
Added citations:
17 USC 503 says:
(a)(1) At any time while an action under this title is pending, the court may order the impounding, on such terms as it may deem reasonable
(A) of all copies or phonorecords claimed to have been made or used in violation of the exclusive right of the copyright owner;
...
(b) As part of a final judgment or decree, the court may order the destruction or other reasonable disposition of all copies or phonorecords found to have been made or used in violation of the copyright owner’s exclusive rights, ...
17 USC 602 provides that:
(a) (2) Importation or exportation of infringing items.—Importation into the United States or exportation from the United States, without the authority of the owner of copyright under this title, of copies or phonorecords, the making of which either constituted an infringement of copyright, or which would have constituted an infringement of copyright if this title had been applicable, is an infringement of the exclusive right to distribute copies or phonorecords under section 106, actionable under sections 501 and 506.
...
(b) Import Prohibition.—In a case where the making of the copies or phonorecords would have constituted an infringement of copyright if this title had been applicable, their importation is prohibited. In a case where the copies or phonorecords were lawfully made, the United States Customs and Border Protection Service has no authority to prevent their importation. In either case, the Secretary of the Treasury is authorized to prescribe, by regulation, a procedure under which any person claiming an interest in the copyright in a particular work may, upon payment of a specified fee, be entitled to notification by the United States Customs and Border Protection Service of the importation of articles that appear to be copies or phonorecords of the work.
It appears that copies of an infringing work may be seized. but only by court order. It appears that import of an allegedly infringing work may be prohibited, but not when copies were lawfully made. But such imports may be reported to the claimant, who can then take appropriate legal action.
