I don't believe there's a way to add in the command you suggest. In the UI, the results are presented as records only in the source, only in the target and "different". Is it "different" records you're wanting to exclude (i.e. where the PK of the record matches one in the source, but the actual data differs)?
If so, you might want to investigate controlling which differences are selected - I've not tried this on some useful data as yet, but something along the lines of:
foreach (TableDifference d in session.TableDifferences)
d.SqlSynchronization = SqlSynchronization.AddSql;
Should make it only generate SQL for records to be inserted, leaving those that already exist (SqlSynchronization.UpdateSql) alone.