It is possible, but you may find recognition accuracy to be less than ideal as the speech recognition engine has to guess what you're saying, rather than just picking from a list of options as with predefined command phrases.
You can input "Hello*" into the "When I say" field of your command, and it will execute each time a phrase starting with "hello" is recognized.
For more information, check the "Wildcards" subsection of VoiceAttackHelp.pdf on page 18 and 19.