Loading/registering multiple xsubs

I am trying to write Perl bindings for a C++ library which has multiple modules.

Assuming we have

Foo.xs
Fo.pm
Foo_Bar.xs

how do I get Foo::Bar to load/boot/import at the same time as Foo on use Foo?

I've tried putting newXS statements in Foo's BOOT:, but I can't seem
to get the invocation right.

Obviously, I could create .pm for each .xs, but as they would be
otherwise empty, there must be a better way.

I see that Glib and Gtk2 use gperl_register_object, but I'm not using glib.

Regards

Jeff