I have been a frontend-focused software engineer for six years and sometimes I just worked on some easy backend work.
Now, I want to apply for a backend role; although I am interested in backend engineering, I do not have enough experience in it. Can I just fake it until I make it for the interview?
You should view any list of job "requirements" as a list of recommendations instead.
As for how you apply, it's simple: just do it! Literally just apply like you would for any other job. If you don't get it, you can try (1) tailoring your resume or (2) getting a referral from someone who can advocate that you're a fast learner.
There's no need to fake anything: just talk about your interaction with backend engineers, your track record of learning quickly, and why you'd be a great fit.
(For any career opportunity, don't disqualify yourself.)
I agree with applying for the role even if you aren't sure of the outcome. Worst case, you'll gain valuable insight about what kind of questions get asked for backend engineers or what kind of experience companies are looking for.
I would also spend some time crafting a compelling story on the backend work that you did, even if you think it's just "easy".
If you are currently working at a company, can you also consider taking on and leading more impactful backend projects at your current job (possibly even switching teams)? That will help to boost your application and provide you with more backend experience.
In general, I would apply to any role where you meet 50%+ of the requirements: Don't filter yourself out!
Unless you're working on really niche backend stuff, you don't need to "fake it" as you can build side projects to create relevant experience. Building the standard REST API that takes in/returns JSONs is super easy to spin up on your own. Here's our playlist about side projects: [Taro Top 10] Building Impressive Side Projects
Another thing you should do is create a backend focused resume where it highlights all your backend work and downplays your frontend work. If you're still applying to frontend roles, you can have a frontend-focused resume (which I assume is just your current one).
If you haven't watched it already, I highly recommend our resume masterclass as well: [Masterclass] How To Write A Stellar Tech Resume That Gets You More Job Opportunities