The strings of classical or nylon string guitars are not ferromagnetic. This means that when used with an electromagnetic pickup like the Submarine no signal is heard. Even the bass strings, which are wound with a metal - usually bronze or coated copper, produce no sound when a Submarine or other electromagnetic pickup is used.
The metals that are ferromagnetic include iron, some types of steel, nickel and cobalt. Steel and Nickel are commonly used in guitar strings. The issues with putting a regular acoustic steel string or set of strings on a classical guitar is that one, it will not sound right; and two, the tension required to bring the note up to pitch will be too great for the neck of the guitar. Most of these guitars do not have truss rods.
String manufacturers Thomastik Infeld make a huge variety of strings and it turns out that one type is ideal for our needs.
The 3 bass strings of Thomastik Infeld Classic S KR116 have a low enough tension to work on a classical guitar whilst also sounding very similar. Their construction is a little unusual - they have something called a 'rope core' which contains multiple steel strands (rather than the conventional solid, single core found in normal wound strings for electric or acoustic guitars or the nylon core found in classical guitars)
The gauges of the 'E', 'A', and 'D' from this set are 0.039", 0.031" and 0.023". They are perhaps a little thinner than regular nylon strings, the 'D' especially feels too then, but the 'E' and 'A' are perfectly serviceable. The picture below shows the 'E' and 'A' from the KR116 set on the left alongside a regular classical 'D' on the right.
They can be purchased as a set or some retailers provide individual strings.
The Equipment used in the video is as follows:
Strings 5 & 6 Thomastik Infeld Classic S KR116 (KR31, KR 39)
Guitar Espana by Barton SL-11