I do wonder though, how would you treat existing armour in relation to crafting?

Generally I find that the armor crafting rules reflect what a manufacturer would have gone through to produce existing armour. There are three ways that existing armour differs from a casual look at the armor crafting rules; Cost, Qualities, and Type.

Type is the easiest to explain - as the base templates for armors are just that; generic descriptions. Most armor sellers will name their armors off of their companies, the materials used, or a publicity tested "buzz-word" that will get people buying. For example, Protector 1 Combat Armor from Dangerous Covenants is described as "heavy battle armor." As far as templates go, it follows the +2 soak and +1 defense of segmented armor, which is where I would classify it. This leads us to the second point;

Qualities - as the basic template defines what a beginning suit has available, it is the generic starting point to work from. Existing armors, however, are examples of merchandise that have gone through extensive and rigorous testing, planning, and many rounds of R&D that have produced a reliable schematic that can be cost effective for a company to market. This means it must have certain qualities that will make it an attractive alternative to the armors of its competitors, while also being quick and easy to mass produce for galactic consumption. This comes after hundreds (if not thousands) of rolls for armor crafting by the various designers and mechanics in the employ of the design firm. As such, it can be assumed that certain things that they incorporate into armors are "trade secrets" not likely to be replicated by a lone crafter. As such, differences in the existing armor to its closest Template are explained. But all that comes at a cost.

The Cost - the Templates assume only the cost of the raw materials an armor crafter uses. The cost of laminate plate, rivets and pins, wiring, flux, fuel for the forge, etc. The existing armors are set at price points designed to reflect a return on investment; R&D, staffing, shipping, facility maintenance, payroll, benefits, aftermarket, publicity and advertising, raw materials, certifications, quality costs, etc. This total cost of production is then factored over the number of units sold until a final cost is determined. This cost, then, very rarely has anything to do with the template, and much more to do with the mass production of the armor. This, then, leads into your next question;

Would you treat all armour entries as templates that can be taught? In case I want to create armoured clothing, which has an effect that effect is not readily available on the list of potential cool (and bad) extra results...

For the most part, I treat existing armor as exactly what it is; manufactured goods. If someone were to want to recreate a suit of Protector 1, I would have them start with the Segmented Template, and make them go through many periods of trial and error, rolling and re-rolling until they get the number of advantages and triumphs needed to pay for all the features of the Protector 1. This might be impossible for them; which is exactly right - my friend is a real life blacksmith and there are many things beyond his skill level to reproduce. If you wanted to create the existing armor of "Armored Clothing" (+1 Soak, +1 Defense, 3 Encumbrance, 1 Hard Point), you could do it two ways;

1) start with a Reinforced Clothing template. Roll 8 Advantages or 3 Triumphs to get +1 hardpoint, +1 melee and +1 ranged defense, ending with a suit of "Personalized Armored Clothing" (+1 Soak, +1 Defense, 1 Encumbrance, 1 Hard Point). You would then be able to absorb up to Despair on the roll to increase the Encumbrance by 2.

2) start with a Deflective Armor template. Roll a single success and no threats or despair and you have a suit of "Personalized Armored Clothing" (+1 Soak, +1 Defense, 2 Encumbrance, 1 Hard Point).

How does option #2 reflect the existing armor? Well, you have to remember that you are crafting armor for yourself, so you're automatically going want it. Big companies need to sell to large number of people with a lot of different tastes. This means that they will need to add features and make it look like something many people will want to buy - and that isn't efficient. The increased encumbrance comes from cosmetic choices that make the armor marketable, but not necessarily practical. Thus, Armored Clothing is explained.