Let’s Encrypt is designed to be integrated with a hosting environment or operating system, and so we’ve stopped spending efforts on situations like this: we would like people who have a hosting provider to be getting their certificates automatically from the hosting provider. One reason that we think it’s not worth it to do things the way you suggest is that you will have to renew the certificate manually at least every 90 days (repeating all of the steps each time). Let’s Encrypt has also hoped to be able to reduce this number in the future, which will be much less practical if there are people routinely getting their certificates by hand.
Normally automated certificate renewal—which is the way Let’s Encrypt is intended to be used—is only possible with operating system, server application, or hosting provider integration of some sort, and we’ve been putting pretty much all of our efforts into trying to facilitate those integrations.
The most likely thing you could use to do this the way you suggest is a web-based client like https://zerossl.com/, which lets you go through the steps in a web browser instead of installing software on the web server. At the end of the process, you save the certificate from the web page. Then, if the hosting provider has an interface through which you can import a certificate, you could import the certificate that you got this way.
If the hosting provider explicitly doesn’t allow you to use external certificates at all (which is entirely possible—it depends on the hosting provider’s decision), then this also won’t help.
Many hosting providers have easy and automatic Let’s Encrypt integrations at no additional charge
and that’s the experience that we’re hoping to get to for everyone who operates a web site or service.