How is the memo field formatted? OR how does it export? Text separated by commas, tabs, or a special character?
If you have Word, you can import the exported Memo field and Find & Replace All "text or tabs or returns" with"your text, tabs, commas, etc"
This will allow you save the word doc as a Comma Dellimited or Tab Deliminated .txt file, open it in Excel, this will show it in a table to easily check it for accuracy
This accurate text file in Excel can be saved as a spreadsheet .xls or leave as .txt file.
Filemaker can import each memo in the Memo Field as a Note in a Table for notes.
If you are on Mac, a free text applcation, called TextWrangler can do the same thing, TextWrangler is by the makers of BBedit.
Backup as you go, of course.
If this is a dbf format import, this is a known limitation of imports from DBF files.
For More Information see: Bug importing dBase data into File Maker Pro 10 Demo (OSX)
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: http://www.4shared.com/file/8orL8apk/FMP_Bugs.html