Yes you can. I recommend doing this before entering a room so you’ll have as much time needed to customize your planes. You can access your hangar as soon as you enter multiplayer mode. You can also access your hangar from the lobby, after successfully entering a room. Take note that once the sortie has begun, you’ll be taken to the match once the countdown as finished, regardless if you’re finished customizing your plane or not.
You also have access to a wide array of customization options as listed below. You can use normal upgrade parts from the main aircraft tree early on but the multiplayer-exclusive parts are undoubtedly more powerful (and expensive)
You can have up to 15 aircraft sets with their individual customizations. It’s recommended to have different aircraft sets such as one dedicated for pushing your SP weapons loadout, damage, tracking, etc and at least another set dedicated to improving your standard Missiles’ power for matches where SP weapons are restricted.
If you enter a room with a cost restriction, only the aircraft sets compliant to the cost limit will be available for selection.