You can assign substitution variable as member but as part of formula in FR I don't think It can be possible.
As Harishkumar Y-Oracle said it doesn't work in IfThen. The reason behind is that IfThen works only on Numbers. It doesn't work on Members.
But, there is a way that you can do it. Depending on where the formula should be, Row or Column, you may need to create two formulas and apply conditional formatting based on the member which row/column should hide / display
Hope it helps