There are two different methods for using makedbm to modify maps if you do not have an input file:
Redirect the makedbm –u output to a temporary file, modify the file, then use the modified file as input to makedbm.
Have the output of makedbm -u operated on within a pipeline that feeds into makedbm. This is appropriate if you can update the disassembled map with either awk, sed, or a cat append.