For Derailleur Hubs
The Brompton 2 speed rear hubs can be upgraded to accommodate multiple individual sprockets, by replacing the freehub bodies. Full conversion kits are also available that include the shifter modification parts, sprockets set, and chain, in addition to the multi-speed freehub bodies.
Aftermarket rear hubs, such as Hubsmith, often comes with multi-sprocket compatible freehubs. And in such cases, full kits are available without the freehubs.