Fitnesse for Appian: Is there a way to use a slim symbol within an expression?

Hi automated test engineers,

Is there a way to use a slim symbol as part of an expression? I realize that we are able to create expressions using Markup Variables with the following syntax:

!define grandTotal {${= ${var1} + ${var2} =}} 

...but we (my team and I) are wondering if there is any way to use a slim symbol (i.e. $totalGridRows) within an expression so that we can verify calculations within our automated script. My first step would be to gather a value from a user interface and storing in a slim symbol as shown below:

|$totalFunds=| get field| Total Funds| value |

After storing the value in '$totalFunds'  I want to add or subtract from that number using an expression. Let me know if this is possible using FitNesse. Any suggestions are appreciated.