Have you installed Rtools already ?
You can modify the path in R only as their is a PATH environment variable acessible.
You can modify at startup using .Renviron file or modify in code using Sys.setenv or the helper function in devtools: devtools::add_path() (see documentation)
I think it could work for what you want to do.
Know that on windows, you can set environment variables for your user only without any admin rights. Typing PATH in windows search could lead to the right menu to modify account environment variable (not system ones)