A harvesting question

One department in our campus harvests a CONTENTdm collection from the library to their Omeka. This collection has many non-DC fields; many of them have been mapped to the same dc.description. The library has been adding "Unknown" and "Not applicable" values for the fields which don't contain content, because we were told that we cannot have empty value for any field:
The reason to use "Unknown" and "Not applicable" is to have placeholders for the harvest. By using them, the Harvester knows what the data items are, by the order, in which they come across. Before adding the "Unknowns", the harvest would skip the blank columns, so there was no way of knowing which fields were skipped or sent. For example, if the "Medals" where blank, then "Achievements" would be in the place of Medals, and all of the other data elements were shifted up into wrong fields.

Is there a way to ask Omeka not skipping the blank field? In other words, is it possible for the library not to add all those "unknown" values but still allow the collection to be harvested by Omeka correctly?