Yes, it is possible. One of the methods is to use the command:

sudo dpkg -i filename.deb

You can also try to install the file via Ubuntu Software/Snap Store application by double clicking on it in file manager (although this usually didn't work for me - but in theory it's supposed to work).

One more method is to use GDebi, it's a GUI installer for .deb files. It is my preferred tool to install non-repository packages. But it's not installed by default, so you have to install it first while you do have an Internet connection.

However, if the package to be installed has any dependencies, you must resolve them manually and install all dependencies first.