The "import" command:

Usage: fossil import --git ?OPTIONS? NEW-REPOSITORY

Read text generated by the git-fast-export command and use it to
construct a new Fossil repository named by the NEW-REPOSITORY
argument.  The git-fast-export text is read from standard input.

The git-fast-export file format is currently the only VCS interchange
format that is understood, though other interchange formats may be added
in the future.

The --incremental option allows an existing repository to be extended
with new content.

Options:
  --incremental  allow importing into an existing repository

See also: export