Can I copy and paste iDevice backups to an external drive?
Yes, you can do this. 

However, if you have a Time Machine backup1 you should already have these backups included, so you could potentially negate the need to copy the backup twice.

Either way, I'd do a fresh backup of your iDevice before proceeding.

1 You should definitely have some sort of backup regime in place, even if it isn't via Time Machine.