I don't think you can create strumming from scratch with any Yamaha arranger.
As far as getting a muting sound in the strum, simply find the strumming pattern and then change to a MUTED guitar voice.
With all the styles available here at the PSR web site (as in thousands) I have always been able to find the correct strumming pattern to add to my custom song styles.
What the guy was doing in the video, I have never been able to do. The closest thing to it would be combining 2 different strum patterns together to get a new sounding pattern. I have also recorded my own second pattern to play along with a preset Yamaha strumming pattern to create a new sounding pattern.
Regards
Drake