Here is a research report which found that you can still get Bacillus subtilis to produce nattokinase even if you skip all the normal steps to make soy bean natto, and rather just make a yogurt with it. It is a lot easier than making natto.
Quote: "Abstract
In this paper, Bacillus subtilis (natto) was incubated to develop a possible functional ingredient in ice cream. A lab-scale culture revealed that incubation in the sterilised milk without dilution and concentration at 37°C for 28 h could obtain ideal growth characteristics of Bacillus subtilis (natto), especially with continuous aeration. Following freezing operation of the cultured milk, survival content of Bacillus subtilis (natto) was at 49–92%, while nattokinase activity was conserved at 62–98% comparing with the initial contents, which indicating a potential for application of natto functional ingredient in frozen milk products
Growth characteristics of Bacillus subtilis (natto) in milk (PDF Download Available). Available from: https://www.researchgate.net/publicatio ... to_in_milk [accessed May 29, 2017]."