One of things that makes R great is the wealth of high-quality add-on packages. As you might expect, there are many of these add-on packages with the ability to import data in a multitude of other formats. Whether it's an arcane markup language, a proprietary binary file, Excel spreadsheet, and so on, there is almost certainly an R package out there for you to handle it. But how do you find them?
One way is to browse the community maintained CRAN Task Views (https://cran.r-project.org/web/views/). A Task View is a way to browse for packages related to a particular topic, domain, or special interest. The germane Task View, here, is the Web Technologies Task View (https://cran.r-project.org/web/views/WebTechnologies.html). You'll notice that jsonlite and the XML package are mentioned on the first page.
The easiest way to discover these packages, though...
