These pre-packaged solar generator lines from Bluetti, Ecoflow, Anker, etc., have nice packaging and convenience wrapped around some (usually small) amount of LFP battery capacity. It was always this low battery capacity compared to most any DIY scheme, and the high cost of "expansion packs", that kept me from going down this convenience path and led me to build a simple DIY solar generator.
My guess is that all of these expansion packs utilize proprietary connections, such that only the vendor's battery pack will work with the vendor's base unit. Otherwise known as "vendor lock-in", and we see it everywhere; it's one of the things you have to consider in the purchasing phase. For Bluetti and similar, this seems reasonable, as their schemes would also cover the engineering/recharging of the external battery packs, something not easy to replicate in DIY fashion.
Unfortunately, the total costs spiral upwards quickly, but that's OK if you really like the convenience and packaging of base + expansion units, and are willing to pay for those things.
The only thing I could find, without going down a rabbit hole of reverse-engineering the proprietary connections, is the various youtube videos showing how to build a battery pack to feed into one of the "shore power" ports (car 12v cigarette adapter, etc.) ... these are limited to low-wattage, slow recharge rates for the base unit, but they do allow it to be recharged from a DIY external battery pack.
Whereas, if one builds a DIY solar generator, it would be easy to swap in larger battery-banks (reconfigure and add batteries). It's probably low down on your list, but an option is to sell the base unit (before you buy into their expansion pack scheme) and apply the money to a DIY solar generator. I'm not sure what the resell value is for Bluetti's and such ...
Hope this helps ...