As far as I know, the Yamaha Style File Format (SFF) doesn't support "half-bar fills," so you have to get creative.
Since a 4/4 bar could be thought of as two 2/4 bars combined, one possibility might be to create or choose a style that uses 2/4 time. The best results might be to create your own style file, so you can make the Main sections sound like they're using bars of 4/4 time instead of multiple bars of 2/4 time.
The only other ideas I can think of involve using two or more Registrations to switch back and forth as desired.
One idea would be to have one Registration set up with a style that uses 4/4 time, and another set up with a style that uses 2/4 time, making sure the Tempo is set to the same value in both Registrations.
Another idea would be to have two Registrations set up with the same style as each other, but different Tempos-- one Tempo being exactly twice as fast as the other.
The thing is, when you save a Style Number in a Registration, you can also save the Style Main A/B setting, so you can recall a Registration that will switch to either Main A or Main B of a desired style, but you can't recall it so that it's playing either Fill-In A>B or Fill-In B>A. So you'd have to use either Main A or Main B of the 2/4 style as though it were a Fill-In, which might involve some careful planning and carefully-timed button pressing.
Also, doesn't the PSR-E3xx series use the numeric keypad to recall Registrations 1 through 9, such that you need to press two buttons to recall a specific Registration? That might also make it a bit tricky to pull off smoothly.
