Yes. I’m already doing it. Have a spare battery (purchased from Purism as part of initial delivery) and an external charger, [MyL5] Australia/New Zealand
While it is possible to swap the battery “live” i.e. while on mains power and hence without shutting down or crashing the phone, at one time at least this would confuse the charging code in the Librem 5. So unless you really need 100% availability, it is probably better to shut down, swap the batteries, and then boot up again.