Bear in mind that the consumers of your code won't actually need to have using statements. Those are there to make their lives easier, so they don't have to type antlr.collections.Foo and antlr.collections.Bar all over their source.
The bigger "impact" (if indeed there really is a severe one) is that the consumer of your code will need a hard reference to the assembly where antlr.collections is defined.
However, if that's documented up front, I honestly don't see it being that big of a problem. It's no different than the consumer of a SubSonic-generated DAL needing references both to the generated DAL assembly and the original SubSonic assembly. (And, quite possibly, using statements as well.)
Dependencies are what they are. There's a reason classes are broken into namespaces -- primarily for organization and to reduce naming conflicts. Not knowing what classes are in the namespace you mention, I don't know how likely such a conflict actually is in your scenario ... But attempting to move the class from one namespace to another, or to hide the fact that such is needed by deriving a blank class from it, is probably not the best idea. It won't kill the consumers of your class to have another reference and using statement.