Lengthen a AC to microUSB phone charger cord?
A couple things.


Charging cables run up to 1.8 amps under the usb spec. I have an answer about it here.
Yes, it will work. Please make sure to use heat-shrink or electrical tap to avoid shorts.
USB runs relatively high speed and 6 feet is further then you might think, this can significantly degrade performance on the communication, not an issue for just charging. There are cables out there that come 6 foot or more.(ignore this if it is a wall outlet to USB charge adapter)
If you are using a wall charger and it is only giving you 750mA you are getting half of the capacity you could be getting. If you phone can handle it, higher end phones will, you will be able to charge in less than half the time. (ignoring the charging characteristics of the last 10% of a lithium battery)


Just a quick list of things that might help. Your splicing the data cables will hurt the data quality and probably radiate some noise. I do not think there are any communication protocols on a harmonic of USB, so it should not matter one bit.