Agree with @scammi here, the best way to be sure is to actually test it.
That said, adding fields to a struct will heavily depend on where you are actually using such structs. If you have a contract variable of the struct type, then you most likely won't be able to add any fields (unless the variable is the last one on your contract). If the struct is contained in a mapping, then you probably will, since structs will be stored sparsely in storage.
Feel free to share your code here if that helps!