You can use cPI like any other constant.
To add 3 to cPI just write Table2.Cell("bot",0)=cPI+3
or Table2.Cell("bot",0) = Table2.Cell("bot",0)+3
I don't really understand your question, because if
Table2.Cell("bot",0)=cPI it's content is 3.14159... and cannot be 15.916 or anything else.
cPI is NOT a variable, you cannot change it's value, thats what constants are made for.
If you need to make different operations with PI you must define a variable or use the cell content.
Best regards.