Hi Toril,
Looking at the PSR2100 Owner's Manual, the section 'Selecting Intro and Ending Types (INTRO/ENDING)' on page 69 (english version) indicates that you can preselect the Intro and Ending played on your 2100 from the MAIN screen display.
On the MAIN display you should see a box next to the 'E' button which shows the currently selected Intro and Ending types.
Press the 'E' button and a window should open which allows you to select the Intro using the 'D' button and the Ending using the 'E' button. I assume that each press of one of these buttons toggles through the available types.
Although the manual does not say so, you may have to press EXIT to return to the MAIN screes after setting your choices.
Once set, pressing the INTRO or ENDING button should trigger what you have selected.
However I assume that this procedure is intended to be used before starting a style playing, and I don't know if you can use it to change the selected Intro/Ending once a style is playing! Even if you can, you would need to be pretty quick to make such changes on the fly!
However it might be worth trying to see what happens!

Regards
Ian