yes... assuming I have understood the question correctly, and you wish to substitute an MSA registration for one of your panel memories in a self made style:

Load up your style and press the panel memory you want to overwrite. Now switch on MSA, select the preset with the registration you want and press the correct variation to achieve the right hand sound you want. Switch off MSA. Now change the style from the preset back to the relevant composer and variation, and correct the tempo if you are not filtering tempo in the panel memories. Now just re-save that one panel memory which will now play your composer rhythm with the new (right hand) registration, then re-save your style to SD or floppy.