Dunno about raw timber, but I'm importing stock bits from both U.S/Can. and Englandicon by mail without problems.
I would think that if the timber was treated and a certificate supplied to that effect, you would not have any problem.
The thing with old gun stocks is that no self respecting woodworm would survive on a constant dose of gun and linseed oilicon, it may tend to give them diarorhea.