if they were tree furns you could but cordilines rely on there root system . the only possible way to get them more full is to cut the growing tip out of them maybe making it grow new multiple shoots but you run the risk of killing them . you could also plant them in the ground as they are potentialy very big plants . you see these plants in pots looking sad everywear if they survived winter as they are sold as if they are ideal pot plants which there not unless you have a huge pot .

In Queensland you probably won't have the problem with frost that Noseypotter is talking about, though. Varieties of Cordyline australis (like this one) don't root from direct cuttings easily, but air layering works pretty well. The alternative, as Np says, would be to cut them down--preferably in spring--feed them, and wait for new shoots from below the cut.

its not a choice i would take thow cutting the growing tip out personaly thow .

Sometimes you can repot into bigger tubs and feed them well, and that will cause them to form shoots from the base and along the trunk, but that is unreliable.

id leavewell aloneapart from like you say putting them in a bigger pot . i like cutting the bottem out of pots as it gives the impression of being in a pot but allows the roots to go wear they like . itsvery efective and ive had some great suxcess doing this . id be inclined to put them in the ground as aposed to a bigger pot but thats personal taste i guess x .