You'll have to separate the left and right channels into separate files to use this method. I would imagine most audio editors are capable of this. If it doesn't have a function for that in the menu options, set the zoom parameter so that it shows the entire file in the space of the window, then select/highlight the left channel only, copy it, create a new mono file--don't forget to set it to 48 khz--and paste the left channel into it. Save. Do the same for the right channel. Then load your mono files into Aften in the order left, right, LFE. Set the channel configuration options to "2/0 + LFE", "LFE channel is present", and "Dolby Surround encoded". Bitrate to 448 kbps.

Other settings for best quality as follows--Fast bit allocation: more accurate encoding; exponent strategy search size: 32; bandwidth: 60; stereo rematrixing: mid/side; block switching: selectively use 256-point; dialog normalization: 31; dynamic range compression: none; apply bandwidth low-pass filter; apply DC high-pass filter; apply LFE low-pass filter.

Using these settings will increase the encoding time somewhat, depending on your computer, but it should still be pretty fast. The sound will be quite good even if left at the default values, but if you can get additional quality out of it, why not?

I'm assuming you're going to use the 2.1 channel format. If you want to make a 5.1 as Satanika did, then of course set your channel configuration as "3/2 + LFE" instead, and be mindful of what order you load the channels.