Most keyboards put the main accordian voices at the end of the organ tones. If you look at the Tone List above the WK-3500's display panel, you should see the accordian tones listed at the end of the Organ Group. If you need a manual, you can download a copy from here:
http://support.casio.com/manualfile.php?rgn=5&cid=008011006
The complete Tone List begins on Appendix Page A1 at the back of the manual.
if you need something with a little more punch than what the regular Accordian voice #54 gives you, try layering it with the Octave Accordian #66.
There are no drum rolls in any of the preset drum kits, but the SE DRUM SET on the Casio website has one on C7 (Key #96) toward the upper (right) end of the keyboard. You can dowload it from:
http://music.casio.com/e/data_ex2/
Click on the green words "Data List" to the right of the words "DRUM with WAVE" and this will bring up a menu of four drum kits. Click on the green words "Download" to the right of the words "SE DRUM SET", and it will download to your computer. You will also need to download the IDES-2 Data Manager software to unpack and install the drum kit onto the keyboard. Back on the main website page, click on the green words "Music Data Management Software 2.0", at the lower right of the page. On the next page, click on "wk3-20en-c.exe", toward the bottom of the page, to download it. Don't bother with the Part-1 through Part-6 downloads. Those download it in small chunks for slow dial-up connections. Be sure to download the Application Manual from the same page. Install the Data Manager software on your computer (PC - NOT Mac) and use it to download the new drum kit from the computer to the keyboard via a USB cable.
The WK-3500 is not Class Compliant, so you will need to download and install a driver for your computer to connect to it:
http://support.casio.com/en/support/download.php?cid=008&pid=20
Look in the DRIVERS section and choose the appropriate 32 or 64 bit driver. If you have a Vista-64 system, use the Win-7 64 bit driver. Casio says that won't work, but I have been using it on my Vista-64 system for over 4 years now. Casio engineering never did understand Vista-64 !
Good luck !