I know forums like this should be exactly where you go for these sorts of things, but... (yeah I'm gonna be that guy)
If you need to ask a forum how to charge a 400V battery pack via manual methods, please don't try anything with it. There are a lot of safeties in the battery pack because they're an incredibly dangerous thing if not properly managed. To bypass those and charge it manually, you need to know what you're dealing with to do so safely, in which case you shouldn't need to ask a forum how to do it.
If you're serious about working up to that point though, start small. Work with smaller packs. Build your own packs, add balance leads, hook a BMS up to them, learn how all that works. Add protection circuits, learn why you need them, learn their shortcomings as well. Then move to the 400V can-absolutely-kill-you-and-set-your-house-on-fire pack once you have a respect and understanding of what you're working with. If you still need a forum at this point, that's actually fine, but you'll have more specific questions to ask about the technicalities and safety of the particular task.