You can reference a variable from an action by using percent signs, ie %USERNAME%. See Using Variables.
To insert a percent sign, use %% - ie "%%Hello" will expand to "%Hello".
Some other characters are also escaped:
All Escape Sequences
Disabling Recursive Expansion
If you have a variable whose contents contain characters marked above, then you can use %!USERNAME% to tell FinalBuilder not to expand any variable references in the contents.
Suppose you had a project variable MySQL, which contained the text "WHERE Connection LIKE 'G%'". If you reference the variable as %!MySQL%, then the contents will inserted verbatim without being scanned for variable references.
Reading Text Files
The Read Text File action has an "escape any variables" option which will automatically escape any variable references found when reading a file.