They can be added for sure. No experience with the Velux "no leak" skylight however. An experienced residential roofer should no no issues adding a skylight. My own opinion is most of those things like, one step, no leak, easi-fit, are all just gimmicks. If the installer is skilled it will not make a difference. If if they are not skilled I would not want them installing anything on my roof.

I have used regular velux skylights multiple times, including one over my shower that was put into an existing shingle roof.

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For my money, there is Velux and then there is everyone else.

Use a skylight-experienced professional roofer and you should be fine.

I have a Velux skylight and there has not been any leaks in many years of having it. I think the best option for that concern is a skylight that does not open. Once you have one that opens, there is more opportunity for infiltration.

Follow Velux instructions; no leaks.

A skylight only helps during the day so you stll will need a lot more artificial lighting . I like all LED 4000K to get the same light as bright daylight some can go on dimmers if needed . I am not huge fan of skylights but thya certainly can be done by a good installer with a good product. Make sure you have some way to control the heat and the UV .