There's no reason why you can't solder the new piece of flex tack to the old right where it sits. I would hack off the long rail on the in-place track so that it's approximately even with the other rail, then slide the extended rail of the new piece back into its proper position. Clean all four rail ends, install the joiners on the in-place track, then slide the ends of the new piece into the joiners. Let the free end of the new flex track fall where it may - don't try to curve it until you've soldered the joiners to all four rails. While staggering the rail joints is generally a good idea, here it's better to keep the joint close to the ties to ensure that forming the curve doesn't pull the rails out-of-gauge. Next time, stagger the joints (or not) but do all of the soldering before forming any of the curve. This is one area where spiking the track in place has an advantage over gluing, as it allows lifting the track during (or after) installation for removal of excess ties to accomodate rail creep as the curve is formed. You could using temporary spiking (not driven home) while forming the curve, then glue the finished curve in place when all the adjustments have been made.
Wayne