There are actually two points you need to be aware of.
First, when composing variables you need to use different delimiters. FitNesse allows three different sets -- (), {}, [] -- just for that purpose.
Second, it is easier to compose in the definition itself, to wit:
!define FileName (${BaseName}.xls)
!define var1 {booking}
!define bookingPeriod $(${var1}June)
And then you can use ${bookingPeriod}
in your code.
I recently wrote a series of articles on practical FitNesse tips, published on Simple-Talk.com. Part 5 specifically discusses composing variables in this manner.