From the documentation for the Import and Export Wizard:
The SQL Server Import and Export Wizard uses the mapping files that Integration Services provides to map data types from one database version or system to another... By default, the mapping files in XML format are installed to C:\Program Files\Microsoft SQL Server\100\DTS\MappingFiles.
If your business requires different mappings between data types, you can update the mappings to affect the mappings that the wizard performs.
You could use something like Process Monitor to find out which file it is using and update it appropriately.