This is a frequently asked question.
See https://cjcoats.github.io/ioapi/M3CPLE.html:
Note that repeated copy-operations may be used to concatenate multiple files that have the same grid and set of variables: for example, if FILE1 has hourly data from 2018017:00 through 2018018:00 and FILE2 has hourly data from 2018018:00 through 2018019:00, a M3CPLE using FILE2 as infile and FILE1 as outfile may be used to copy the 2018018:01-2018019:00 data from FILE2 to FILE1. Repeated runs of this sort may be used to construct month-, year-, or even decade-long files from a (large) set of single-day files…