Here is a little macro for helping with fixed-length data.
But the inserted colums are not virtual as you wished in your original
posting, they are real...
(So you have to delete them afterwards. That can be done with a
a) the fixed length data (line# only shown for better description):
b) insert a ruler line (e.g. as new line 1) marking the field limits:
1 x x x
c) position the cursor in that ruler line
d) execute the macro
e) the result: (the "_" stands for the configurable inserted char.)
1 _x _x _x