At the risk of starting WW3.5, is it possible to add a mains storage
system
to an existing (Bosch Worcester Greenstar) combi?

Yes. You can add anything to a combi that could be added to a conventional
boiler. A combi is just a conventional boiler with an additional
instantaneous hot water heat exchanger and associated controls.

You'll need a pile of extra controls, programmers and valves, as would be
required for a heating system using a conventional boiler. These are
frequently packaged into convenient kits with substantial discounts.

And if so any recomendations?

There are two main contenders. Both are similar in purchase cost. Both
vastly superior to a combi boiler in performance, assuming your mains can
supply the water.

a) An unvented hot water cylinder. This provides the ultimate flow rate,
assuming your mains can supply it. It requires extra safety controls, which
mean that you need a specific safety inspection for building regulations or
a specifically qualified installer. Example: Heatrae Sadia Megaflo.

b) A heat bank (or thermal store, which is similar). This provides good flow
rates (again assuming good mains supply), but not as good as the unvented
cylinder. It is inherently safer as the hot water storage itself is at
atmospheric pressure, so is not subject to potentially dangerous pressure
rises when heated. This makes is much easier to install. Example: DPS
Pandora (also available with complete CH+DHW 'Y' or 'S' plan system controls
and 2 channel programmer ready mounted to the cylinder).

I've just bought a DPS Pandora, but it won't be connected for a week or two.
It looks the part, though. We've managed to hoist it into the loft, where
there is no convenient overflow or pressure relief outlet due to a planned
dormer extension.

Christian.