You can, but it takes more work because a diverter needs somewhere to put the heat, and a combi boiler heats water on demand without a tank. To use solar surplus for hot water you'd typically need to (re)install a hot water cylinder and an immersion, then a diverter to feed surplus into it. Several Irish solar owners have gone down this route after previously removing their tank, often when upgrading a bathroom or moving off an electricity-hungry electric shower.
There are plumbing wrinkles to be aware of. Properly, a pumped hot water solution should be fed from an attic buffer/storage tank rather than pumping directly off the incoming mains, so retrofitting a tank can mean more than just dropping in a cylinder — you may need an attic tank and pump arrangement. That said, installers who do combi conversions describe workable setups (for example using an attic tank to feed a pump that supplies both the house cold water and the combi), so it's a "get a competent installer to design it" job rather than an off-the-shelf add-on.
Weigh the cost against the benefit. If you only want summer hot water and your combi already does the job cheaply enough, the payback on a full tank-and-diverter retrofit can be slim. But if you're already renovating, or trying to ditch an expensive electric shower, adding a cylinder and immersion so you can capture solar (or cheap night-rate) heat can make good sense. Get a couple of installers to spec it for your specific plumbing before committing.