Not a variable directly, but you can use a token in the parameter box. So, you can set a text variable (called, 'myVariable') to '-someParameterValue', and then put {TXT:myVariable} in the box. {TXT:myVarable} in this case will resolve to, '-someParameterValue' when it is run.
Tokens are used (versus straight variables) so you can have multiple items in the box that will be parsed out into a string.
Hope that helps some!