Yes, that is possible.
Press F1 while VoiceAttack has focus to open VoiceAttackHelp.pdf in your default PDF viewer, and search for these sections:
"Dynamic command sections"
"{TXTNUM:variableName / value}"
"Variable Pause Screen"
"Key Press Screen"
Keep in mind that commands containing numbers can have a large phrase count, so it's always recommended to only create the phrases you're likely to need (E.G. if you don't need a one-second resolution, use a multiplier).