How can I extend a floating floor that was previously terminated at a wall?
The only way this would be possible, in theory, would be for you to slide the truncated boards out of their positions lengthwise, and replace them. The trouble is that most flooring 1) has a core composed of particle or fiber composite, which has very high friction, and 2) has joint surfaces coated with a waxy material to create a water-resistant seal, and 3) requires that the end joints be tipped into place. Combined, these characteristics would make for a very difficult job of sliding along the length of the boards. 

You'd then have to drive the new, full boards into position without damaging them and somehow engage (or modify or remove) the end joint fittings. All that typically proves impossible, or at least prohibitively impractical. 

That said, many manufacturers offer kits and procedures for replacing individual boards if they get damaged. These sometimes involve a special joiner strip that allows you to engage the tongue or groove of the adjacent board in a manner required to fit it out of sequence. This may be one possible avenue. 

Finally, most floating floors have a length or run limitation due to expansion and contraction. If your final run would exceed that number, typically somewhere around 30 feet, you shouldn't attempt this anyway. 

The bottom line is that you'll need to investigate the specific product you're using for these options, make sure you understand the assembly process, assess your own skill level and the project parameters, and then either go for it or don't.