Yes, this works. But you cannot indent the lines. AsciiDoc is left-aligned, so if you want the lines to be interpreted as AsciiDoc, they need to be flush to the left margin.
If you do want to store the lines indented, then you need to set the indent attribute on the include directive as follows:
include::filename.txt[indent=0]
That will strip the indentation before reading the lines.
-Dan