Communicator 5 does, in fact, have abbreviation expansion. The abbreviation expansion does not to be set up. You cannot choose your abbreviation, rather it is automatic. If you type the first letter of each word in a phrase then that phrase will appear in one of the phrase prediction buttons on the keyboard. See the example below.
When you type in the letters "ic" you will get phrases such as "I can't stand", "I can't think about", and "I changed my mind". When you select any one of those phrases the letters "ic" in the message window changes to the phrase that is selected.
Another example, instead of typing out the entire phrase "I changed my mind" you can simply type "icmm".
Then select the appropriate phrase prediction button to replace the abbreviation with the phrase in the message window.