Maybe a separate compartment.
People have drawn fan art of a 055A with giant VLS amid ship for DF-21D sized AShBM, however I don't think that's really worthwhile for a 055 upgrade.
In recent years there's been a "全能舰" or "All Energy Warship" concept been promoted by Admiral Ma "Anything can be launched electromagnetically if you have a big enough railgun" Weiming where a nuclear powered IEP warship has all of its weapon system relying on electrical power instead of stored chemical energy:
- railgun main guns 10-200km range
- laser CIWS 10km range
- reloadable electromagnetic catapult launcher for missiles 200~600km range
- electromagnetic catapault launcher for ASW torpedoes
- anti-torpedo coilguns
The idea is that because nuclear power is so much more dense than chemical energy, by using nuclear power as much as possible one could fit more stuff into a ship hull.
With the electromagnetic missile catapult I think the idea is to boost a missile high up with the initial sling, to function as a replacement for first stage rocket. So a bit like the US Strategic Long Range Cannon idea. In such a system it may make sense to sling something like upper stages of DF-21D or DF-26B or even the DF-ZF.