I am adding this script to our todo list, but to be honest I have no clue at the moment, when we can possibly take the time for it. We will consider such conversion tools in the future, though.
For the time being, you might want to take a look at various conversion tools between mysql and postgres, like: http://wiki.postgresql.org/wiki/Converting_from_other_Databases_to_PostgreSQL#MySQL
Please note however, that I don't know how good they are (this is not a recommendation, I'm afraid).
In case you want to try, it would be best to try on local installations, not online.
In case they are helpful enough to import the database, at the very least you should also run the upgrade script (for postgres) to your SMF version after trying to import the database in Postgres. (meaning you need to upload the files upgrade.php and upgrade sql for postgres from a large upgrade package, and run the upgrade.php). it should, among other things, try to add the necessary functions in Postgres that are needed by SMF's queries.