We actually implemented a Polygon historical data integration in a development branch earlier this year but abandoned it when we obtained the minute US stock dataset. The abandoned integration mirrors the Interactive Brokers integration, collecting data from the API and saving it to a SQLite database. Basically exactly what you're asking for.
I don’t plan to make this feature generally available because the only purpose would be for collecting historical second or tick data, and I don’t want to mislead customers by implying that they can save massive amounts of data to a SQLite database and have good performance. Many trading platforms overpromise and underdeliver, and I think that's a bad business strategy.
That said, if you are interested in using this integration, we could explore the possibility of dusting it off and making it available on a custom basis for an additional fee, with the understanding that you would be responsible for loading a small enough amount of data to maintain the level of performance you require. Reach out privately if you’re interested.
As for loading arbitrary custom data, that is frequently requested, and QuantRocket will surely support it in some capacity in the future, but we're not there yet. The challenge is that many users like QuantRocket because it makes working with data easy, but it is basically not possible to create an API that can deliver that same ease of use against "arbitrary" data. An API must be designed in tandem with the underlying data, not operating blindly. This doesn't mean a platform can't support custom data and still deliver good user experience, but it means there is no one-size-fits-all solution and we are still sorting out the best mix of solutions.