Yes and No...
You can't 'bulk' copy but if you want to copy a few contacts:
1. Go to contacts and copy the number (highlight number and then menu+c)
2. Go to the SIM Book application
3. Tap ADD
4. Input a name and tab to the NUMBER field.
5. Paste the number (menu+P)
6. Tap OK
Done
You will have to do this for every number you want to put on your SIM card. I know of no other way
Or, if you have a Nokia, Motorola or Ericsson that can connect to a desktop computer:
http://kb.palmone.com/SRVS/CGI-BIN/WEBCGI.EXE?New,kb=PalmSupportKB,CASE=obj(36401),ts=Palm_External2001