This is a pretty old thread but the blanks when importing a mixed worklist is from Microsoft jet engine trying to guess the type of data in the file, by default it only looks at the first 8 rows I believe. This is turned on when you import mixed types.
By changing the registry here:
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Jet\4.0\Engines\Excel.
Then setting:
TypeGuessRow from 8 to 0
This should allow you to import mixed types without blanks. Also FYI the Append Mode does not use the TypeGuessRows function and can be used to both read or write to a file.