One way to do this is through sltest.harness.import, which allows you to import a stand-alone model as a test harness model.
If you created a harness using the "parent" model as the unit under test (similar to a model reference), use the "parent" model name as the 'ComponentName' in sltest.harness.import
Here is a simple example:
mainModel = 'UnlimitedFreeTacosModel'; % free tacos need not only be on Tuesdays
harnessModel = 'UnlimitedFreeTacosModel_harness'; % ensure this works on all the other days of the week
load_system(mainModel)
sltest.harness.import(mainModel,'ImportFileName',harnessModel,'ComponentName',mainModel,'Name',harnessModel(1:end-4));
save_system(mainModel)
