There isn't a formatting trick in one model, unless you define the numeric field as thousands, export it as a fixed width text file, and open that up as a new report and trap it as a character field. But that's too much like hard work.
You can do it though, using Len, Substr, and Str, as long as the number of decimals is consistent. Substr([String];4;3) returns positions 4,5 and 6 of a string - easier than nesting lots of Left() and Right() functions. Str([Number];decimals;length;fill character) will turn 123.45 into "0000123.45" with a length of 10 and 2 decimals and "0" as the fill. I think I'd use "x" or something non-numeric as the fill, in order to be able to use Replace() at the end when I want to clean up the output. Replace ("xx3,456.78";"x";"") is easy, but you'd want to make sure "x,987,654.32" didn't end up with a leading comma, but that can be handled using a nested Strip(Replace ([Ouput];"x,";"x");"x").
The easiest way to make [Output] is just to copy and paste, making sure that you've converted the number to the right length of x's:
Hope this helps,