Is expanding the research of a group into machine learning as a PhD student risky?

I would ask about having a co-supervisor. Having access to esteemed DL researchers is great -- but they will have limited time/interest in helping you if you are not "formally" their student. If you manage to find someone in this role, I think your position is just about perfect.

If you don't manage to find someone in this role, I have three main concerns:

• You will spend a ton of time re-inventing the wheel. For example, can you train a CNN on ImageNet from scratch? There are a lot of caveats needed to obtain state-of-the-art results (e.g., dataset augmentation, regularization loss, etc.), and you will likely rediscover them one-by-one (or, use a black-box model you don't really understand). A DL expert would likely already have working code and could explain it to you, allowing you to jump right to the research. (Yes, there are open source codes...but in my experience, they all require a lot of work to be both transparent and accurate.
• Mathematical rigor. It's easy to just learn ML/DL at a "technician level" -- but as a PhD in it, you should really understand it a mathematical level if not a theorem/proof level. It can be difficult to do this on your own.
• Problem selection. Your medical advisor will likely find it super novel to run existing techniques on medical images. There may even be a novel application here, on the medical side -- but on the ML side, this is not really interesting, it's just a straightforward application of one technique to a straightforward problem. This is maybe OK if your interest is entirely on the medical side -- but if you want to do something also interesting on the ML side, you would essentially be on your own to come up with something. That will be difficult to do (for the first time) without advisors on both sides.

Those are the main blind alleys I see. Of course, there is also a ton of upside -- this sounds like a very interesting, prestigious position that would position you well for an academic career. Only you can judge this tradeoff.

Do you want to design a tool that can build many things, or learn how best to use the available tools to build a house?

Do you want to do a PhD in machine learning or are you trying to use machine learning to solve problems in medical imaging?

In the first case I would agree with @cag51. Without a Deep Learning supervisor, it would be challenging and also unlikely your PhD would reach its full potential.

However, if you are more interested in finding novel and practical uses for existing machine learning techniques in order to improve the field of medical imaging then the lack of specialist supervisor is less important. There is a startling amount of low hanging fruit which requires only a broad conceptual understanding of machine learning combined with domain-specific expertise (e.g medical imaging).

After your first paper/project you will no doubt discover a host of problems that are specific to your domain area which require further research and in-depth knowledge of the domain area which can be provided by your primary supervisor.

It could be a great opportunity to help the field take advantage of benefits provided by machine learning in a very applied and practical way as well as carve out your own niche in academia.

Sounds like a great fit, with some options for different paths post-Ph.D. along with some fallback if things don't work out perfectly. I wouldn't be super concerned about having all kinds of supervision by a deep expert. It is common for grad students to do their own work without significant apprenticeship by the "advisor" (grant writer). As long as you are careful to look out for yourself by sticking to tractable problem(s), it should be fine.

In addition, you seem to have thought things out and expressed them well. And some of your comments (like department work in signal processing) show enough awareness that you seem to be able to look out for yourself and drive your own research.