You would have to find a way to permanently limit to hold only 5 rounds BEFORE entering country.
The faux extension just unscrews like the magazine cap does. You could try and get a +2 extension, and epoxy a cylinder of something (plastic block or something) into the tube so it functions just like the faux extension.
You could TRY and get ahold of a full length magazine tube, but it would need to be permanently limited to 5, or riveted through (not exactly good looking).
If you did that, it would be impossible to get the follower and/or spring out should the gun ever need any maintainance to the magazine tube.
As far as things go here, its best to just leave the fax extension on... It looks different, but IMHO it kind of matches the handguard and pistol grip, and is a little more artfully done.
If you are dead set on getting the look of a full length tube, you could always fill the ribs with epoxy, and then use a high grade epoxy-based paint and paint the whole faux extension black to get a seamless look.
ETA:
Yes, 4 or 5 parts (depending on if the ATF counts the pistons once or twice as parts)
(it has 15 parts according to the 922r "parts Table", assuming you count the pistons as 2 parts. Any imported gun that doesn't meet the sporting purposes has to have no more than 10 imported parts.)