You'll need to use an Expression. In the Variables window, click on the Variable in question and hit F4 to bring up the Properties for that Variable. Set EvaluateAsExpression to True so that SSIS will know at runtime that it should re-evaluate the value of the Variable each time it is referenced:

Then, click in the Expressions property field just underneath the EvaluateAsExpression property and click the ellipsis on the right to open the Expression Editor. There you can enter your expression to build your SQL statement using any Variables in your Package: