TL;DR: Fake it until you make it. Position yourself as a consultant who can solve business problems with technology. Don't just say that you are willing to learn new skills, but also prove it by doing great work along the way using your other skills.
I'm a UX designer with my own company. Many designers deal with impostor syndrome - even those of us who are great at what we do. And there is a temptation to have a sense of inferiority compared to other designers or compared to people who have skills that we don't. Technology changes fast for us too.
Second, position yourself as someone who can do great work. You might have different skills right now than what your clients are asking for, but keep the quality of your work high regardless of what you are working on, and your clients will understand that when you tell them you're willing to learn something new, you'll still work at a high quality.
Since you said you already have a solid hourly rate, I assume you have done this to some extent and that you are bringing value to your clients. If you meet too much resistance with not having the skills, teach the skills to yourself on your own time and create a personal project for your portfolio. I would say at that point, you could include js/jQuery in your skill list. That will also show clients that you have a strong enough interest in your craft to stay on top of new technology.
I no longer believe that "when can I start charging for this?" is the right question to ask. I switched from working as a software developer in a traditional corporate setting to doing UX design as a full-time freelance business owner. Big leap.
So when I first started freelancing, I did a lot of free work. I thought that would get me exposure and put work in my portfolio. The clients gave me some thank yous and recommendations, and I was grateful for that. But ultimately, the inbound leads that it generated were from other people who told me I was too expensive even though I was quoting them less than the average hourly rate for UX designers in my area. Free/cheap work attracts other free/cheap clients.
I'm still paying for the decision I made last year to work for free. I wish that if I had done any free work at all, that it had been a very small project instead of a series of large websites.
There's never a reason for a working professional to work for free, unless a) you have other income paying your bills, b) you are doing this as a volunteer for an organization you care about, who can't pay you, and c) there is a very clearly-defined project scope. Outside these parameters, if you promise to finish a free project, it's just a gift that keeps on taking from you.