How do I add a new custom attribute to an existing geom::Source?

for example if I have a geom::Cube, and I want to add a custom vec3 attribute , What is the best way to do that? Im looking through the geom::Modifiers and I see Remove but no add? Do I have to subclass form geom::Cube and override

uint8_t		getAttribDims( Attrib attr ) const override;
AttribSet	getAvailableAttribs() const override;
void		loadInto( Target *target, const AttribSet &requestedAttribs ) const override;

that seems like too much work?

Perhaps geom::Constant with a CUSTOM_* attribute is what you’re after?

geom::Constant puts the same value for every vertex. I want a different value for every vertex. I have an std::vector<vec3> which has my data. I just want upload that as attribute data.