Currently, there are util methods on org.jboss.weld.environment.se.Weld allowing to manually register a portable extension with Weld.
There is no such counterpart for build compatible extensions(BCE) which might be handy. IIRC, we now always try to discover BCE and that's the only way to use one.

Note that similar concern is for pure CDI API SeContainerInitializer which also allows to add a portable extension but not a BCE.