Quite easy, just download the package from the CRAN site e.g. https://cran.r-project.org/web/packages/imputeTS/
You can either download the "Package source" or the "Binaries" (must be your operating system.
If you are using R Studio, there is then even a menu item for installing the package - just select the file you downloaded and here you go.

If you are not using R Studio, just provide the path to your downloaded archive in the install.packages
command.
The only problem is, you also need all the dependencies ...;)
So you ideally already have them and just one package is somehow missing - otherwise this can get quite time consuming to download all the dependencies - because the dependencies itself also usually have dependencies themselves...
In this case the miniCRAN
solution Roland linked in the comments might be an idea. ( Offline installation of a list of packages: getting dependencies in order ) But didn't try the miniCRAN myself yet - would be interested how good this actually works.