A tool to convert PostgreSQL database dumps between versions?
Are there tools or options to create a pg_dump file that is compatible to earlier versions of PostgreSQL  


No there are no options or tools to create a backwards compatible pg_dump files.  

Depending on how far back the version the data needs to be ported to the dump file may work there is no guarantee   

A work around this limitation  would be run pg_dump using the -P --plain and --schema-only  modify the  resulting SQL file to make it compatible.

Restore the file. 

Then run pg_dump again using -P --plain and -a --data-only options then restore the file.