There are a few ways, most are similar to this :
You can download the package, unzip or decompress, go to that folder
and run: apm link that will create a symbolic link from that package
to your ~/.atom/packages folder.
But ensure that you install all dependencies require by the package also, as if you dont have internet connection then you may encounter issues.
There some good information here i will quote for your convenience that outlines how to handle this :
When you manually download and extract the zip file you need to run
apm install in the package's directory afterwards to pull in any
dependencies. This will download all dependencies and place them in
the node_modules folder and recursively pulls in their dependencies as
well. Since this is not possible in your environment, you'll have to
do that manually.
Recursively go over each package.json file. If it lists a package as a
dependency search on npm6 for the package and follow the link to
package's github page to read its package.json and repeat the whole
process.
Hope this helps in anyway. Lemme know if i can help further once you have tried.