According to this page, it's easy:
Then paste and modify an example like this one. This page (or
man 7 mdoc) explains the formatting options:
.\" Manpage for nuseradd.
.\" Contact firstname.lastname@example.org to correct errors or typos.
.TH man 8 "06 May 2010" "1.0" "nuseradd man page"
nuseradd \- create a new LDAP user
nuseradd is high level shell program for adding users to LDAP server. On Debian, administrators should usually use nuseradd.debian(8) instead.
The nuseradd does not take any options. However, you can supply username.
.SH SEE ALSO
useradd(8), passwd(5), nuseradd.debian(8)
No known bugs.
Vivek Gite (email@example.com)
Then simply gzip and copy your new man page to the proper man section:
1 Executable shell commands
2 System calls (functions provided by the kernel)
3 Library calls (functions within program libraries)
4 Special files (usually found in /dev)
5 File formats and conventions eg /etc/passwd
7 Miscellaneous (including macro packages and conventions), e.g. man(7), groff(7)
8 System administration commands (usually only for root)
9 Kernel routines [Non standard]
As the example is an admin tool, it goes in section 8:
cat nuseradd |gzip > /usr/local/man/man8/nuseradd.1
Or have people read it from another location, e.g. locally: