If it's saved as Raw Source, it's not a .eml file anymore. Unless Apple changed them again in 10.5.x, Mail saves messages as .emlx files.
So, you can download this tool,
emlx to mbox Converter to convert individual messages or whole groups to mbox files and import those into Mail (you need to choose Other when the dialog window asks "Import data from").
If the Mail file format really did change in 10.5.x to .eml, here's a similar tool to convert multiple eml files to mbox format:
eml2mbox. This is a Ruby script, not an application.
Mulder