Can you have such a thing or is this one of Doctor Evil's fantasies?

If you can...how would you do it?

You'd do it the same as any secondary circulation loop. It doesn't really
matter what the heating appliance is, provided there are no pressure
reducing devices such as TRVs or air breaks (i.e. cold water cisterns)
actually in the loop. These may be before the loop, however.

However, not all combis would be suited to a secondary circulation loop.
They work better on a storage based system as an instantaneous combi will
cycle with much greater rapidity than a system boiler off a storage
cylinder.

Christian.