You can use it to find whatever you want, as long as the data is entered in the store locator itself. If you want to use a different source ( post type ), then you will have to make some changes in the code for that to work, and ( this is likely the problem ), have the coordinates available as well. Without the coordinates you can’t make a search.
What happens is that a user on the front-end fills in a location, that location is turned into a set of coordinates, and the DB is search for other coordinates that fall within the set radius. So without the restaurant coordinates, it’s not possible to search for anything.
I don’t have time to test it out, but I can point you in the right directions. The main thing is the query that searches for nearby locations once a users started a search. The SQL query by default uses the ‘wpsl_stores’ custom post type, instead of ‘restaurant’. This can be changed with the wpsl_sql filter, but you will also have to make sure the correct meta fields are used to get the coordinates for of the restaurants.
So maybe it’s easier to move the locations details from the restaurants in the WPSL plugin itself, instead of making the code modifications to make it work with another custom post type?