Skipping one option while mapping to Salesforce

Hi all, Has anyone encountered the following dilemma: We have a matrix question with several statements and with one of the options being N/A. In SF we have a numeric field, and we don't want to change the field type to map the N/A there. We want the field to be empty in case there is an N/A option selected by the respondent. How can we do it? In the actions I don't see any kind of easy solution for this. Thank you in advance!