Prompting the CHGMSGD command limits you to 510 characters for Second Level Text, but the command allows up to 3000 characters.
Use either command entry (F10)under PDM and take F11 for full display, or write a CL program to execute the ADDMSGD (Add Message Description) or CHGMSGD (Change Message Description) command.
The CL approach has the advantage of being able to make a small change to existing text, without having to rekey the entire message. For easy retrieval and maintenance, name the CLP the same as the message ID.
PGM CHGMSGD MSGID(xxxnnnn) MSGF(Library/File) + SECLVL('This is the first line + of the second-level text + (insert up to 3000 chars) last line of the message text') ENDPGM