Convert null as String into Null as value

Hello,

I'm kind of newbie in SoapUI,could someone help me to clarify if is possible to convert a null (String) to a null (value) when I try to insert values into a Table?

Here is the structure of the project

What I want to achieve is the following:

Inside the TestSuite named "Inserts", I want to group all the JDBC inserts "Insert1", "Insert2", "Insert3" and try remove the "Inserts" steps since I have some of these that have commons values. Also I have some inserts that have "null" values in the columns, but when I try to call those steps in the "Inserts" TestSuite, these values are inserted as null as String. Any ideas?