From the help database:
"The Solicit Code Import is used to create solicit codes for constituents or to create an import file of constituents’ solicit codes. Solicit codes for constituents can also be created through a Constituent Import.
*Note: This import type is only available to add additional solicit codes to a constituent record, not to update existing.*"
One more reason to use attributes and not solicit codes. I have not used solicit codes for several years.