As you know the .fld is SCIRun's native format, and unfortunately I don't know of any other program that can produce a .fld.  My suggestion would be to read in you mesh using the "ReadField" module.  There is a drop down menu that allows you to select several different format types.  The format will be converted into the "field" format that SCIRun uses.  You can then write it out as a .fld with the "WriteField" module.

I hope this helps,

Darrell