Not an area I know much about I'm afraid but your post prompted me to do a little research. You have probably done the same.
This (see the section for file types)
suggests that the dbt file is a text file used for memo fields.
That has me wondering whether your snapshot from sql is output to a non-compatible file type that Monarch does not cater for.
Have you tried opening the snapshot file(s) using Excel or Access or something else that should have dBase file format read capability?
Does your snapshot output file creation allow you to specify different versions of dBase for "compatibility"?
Like Grant, I've never used these types of files. In the help, I see that dBASE III, IV, and 5.0 are listed as input options.
Just a thought - did you use Open Database instead of Open File in Monarch? I see that if you do select a data source and browse for a local file, under file types *.dbf is an option for DBF Files.