Sync sic code (account) to salesforce

I see that Marketo does not sync Salesforce Account object fields over, just from Salesforce to Marketo. We have a Reachforce integration that enriches with SIC code and other fields and I am wondering if it will push that information into Salesforce or if (which I expect to be more likely) the Salesforce NULL value will actually overwrite the Marketo value and we will lose it altogether. Please help.