try passing the null value (access keyword) as a string in the sql query
i.e.
sql1="INSERT INTO tblMiTabla (Col1,Col2,Col3 ,Col4)
sql2="VALUES (" & var1 & "," & var2 & "," & IIf(IsNull(var3 ),"Null",var3 ) & ")"

where var1,var2,var3, var4 are variable names...