How can I add a handset - Uniden 2.4 GHz Cordless Phone System

Yes it can be done, the instructions are in the phones user manual. Here is a link to help you add the handset. VTech DECT 6 0 CS6219 User Manual Add And Register Handset

just add the extra handset then follow the set up procedure as explained in the user manual
the base set has to recognize each new handset and allocate it a hand set number
all explained in the user manual

GOOGLE! GOOGLE! GOOGLE!
emerson EM7000 manual

Google
First thing on the list!

look for Registering your handset.

if you can get the exact model as the existing hand set it should not be a problem
just follow the installation process to get the base to recognize it so that it connects

Here is the manual:

http://www.telstra.com.au/help/download/document/telstra-v580a-290409.pdf

Apparently,
d1384 page 7 of http://www.uniden.com/content/ebiz/uniden/resources/ownersmanuals/D1384om.pdf

Finding the Handset
With the phone in standby, press
FIND on the base. Each handset will beep for 1
minute. To cancel, press
FIND again or press any handset key

I think you all totally missed the question. The phone set comes with the base and 2 extensions. He is wanting to BUY a 3rd extension for this unit, if I am reading this correctly. I have the same phone set and I have not found anywhere that sells additional handsets. Sorry.

You can add handsets from Panasonic that are from associated models (see manual). Handsets from another company would be problematic.

Add more handsets to what model Uniden phone?

1. Place camera and handset no more than 1 foot apart.
2. Select "Device" on handset and highlight "Add."
3. Press and hold button on camera. When green light starts to blink rapidly, press right on handset while continuing to hold camera button in.
4. When handset says "Please Wait," camera button can be released. "Successful" will display on handset screen when camera has been added.