Looking at the user manual for your product, it does not possess HDMI inputs or outputs. However, you could run your audio through the device:
1. Run RCA cables (red and white) from your Roku to the audio in on your Panasonic unit.
Roku audio output-->Panasonic unit audio input
2. Run an HDMI cable from your Roku to the HDMI input on your television.
Roku HDMI output-->Television HDMI input
When using the Roku, turn down the volume on your television and turn on your Panasonic unit for audio.
The panasonic does have an HDMI output..
The Panasonic web page has your model listed, but the first manual shows the SC-HT40, not SC-HT740. I did find the right manual and you are correct, there is a single HDMI out connection; which carries audio and video to your television. However, you do not have an HDMI in connection. Therefore, you cannot route your audio and video for the Roku through your Panasonic unit. However, if you want just the audio from your Roku to play through the Panasonic unit, you can do that. The guidance I gave above, will work for your situation.
If you already have an HDMI cable going from your Panasonic unit to your television for DVD playback, the setup gets kind of ugly. In short, you would place an HDMI switch between the Panasonic unit and your television. One input into the HDMI switch would be from your Panasonic unit, a second input into the HDMI switch would be from your Roku. The output from your HDMI switch would feed the single HDMI input on your television. So it would look like this overall.
Panasonic unit HDMI output-->HDMI switch input 1-->HDMI switch output-->Television
Roku HDMI output-->HDMI switch input 2-->HDMI switch output-->Television
Roku Audio output-->Panasonic unit Audio input
When watching a DVD on your Panasonic unit, you would choose 'DVD' as the source on the Panasonic unit, select input 1 on your HDMI switch and the HDMI selection on your television.
When watching your Roku, you would choose 'AUX' (or the appropriate title assigned to the audio input) as the source on the Panasonic unit, select input 2 on your HDMI switch and the HDMI selection on your television.