Yes you can install .Net Framework 3.5 in Windows 8 offline. The .Net 3.5 package is already included in the ISO but installing it requires an elevated command prompt which means you have to run the command prompt as an Administrator and run Deployment Image Servicing and Management Tool (dism.exe)
Click the image for larger picture
as you can see on the image, Windows 8 installer has x86_netfx35~~~
So after running the command prompt. Type
dism /online /enable-feature /featurename:NetFx3 /all /source:<w8_setup_drive>\sources\sxs
Then wait until it completed successfully.
Post Update December 11, 2012
Some users having a problem with the installation. Here'a useful link to resolve the problem
Error codes when you try to install the .NET Framework 3.5 in Windows 8 or in Windows Server 2012
http://support.microsoft.com/kb/2734782?wa=wsignin1.0