Two ways to do this:
Set integer [Variable2] value to 2
Set integer [Variable3] value to 3
Set integer [Variable1] value to the value of [Variable2]
Set integer [Variable1] to [Variable1] plus [Variable3]
Set integer [Variable2] value to 2
Set integer [Variable3] value to 3
Set integer [Variable1] value to the converted value of {EXP: {INT:Variable2} + {INT:Variable3}}
Press F1 while VoiceAttack has focus to open VoiceAttackHelp.pdf, which has more information on the {EXP:} token.