There are no laws whatsoever telling you how to bring in pistol magazines that hold more than 10 rounds or mandating the use of an FFL.
What we have are laws prohibiting them.
(c) The manufacture, possession, sale, barter, trade, gift, transfer, or acquisition of detachable ammunition magazines with a capacity in excess of ten rounds which are designed for or capable of use with a pistol is prohibited. This subsection shall not apply to magazines originally designed to accept more than ten rounds of ammunition which have been modified to accept no more than ten rounds and which are not capable of being readily restored to a capacity of more than ten rounds.
If a mainland dealer is willing to sell you mags, and you are willing to receive them, power to you! Magazines aren't controlled under Federal law (at least, not yet) so it's legal for them to be shipped. There's no law on the mainland telling them not to sell to Hawaii, so it's legal for the vender to process the order. The only person breaking the law will be you by possessing them for whatever period of time it takes you to get them blocked.
But no one need ever know. It's not like UPS or USPS or FedEx will be turning over records of deliveries to HPD.
Likewise, you can buy them in person, stuff them in your luggage (even your carry on, technically, although I sure wouldn't try it!) and bring them home. You're not a lawbreaker until the plane lands, but again, these are unregulated items, so there is no reason any law enforcement agency would ever know.
I've heard it argued that there is no law against having the parts. the statute does not address components, merely the whole. So having a boxfull of parts wouldn't be illegal, while assembling those parts into a boxfull of mags would. Of course, IANAL.