You can download freeplay ROMs for Pinball Champ from David Gersic's site here:
http://www.zaccaria-pinball.com/gen2/pinballchamp/roms.html which will give you the option to switch between paid or free play using the normal programming of the table on the coin meter setting (Option 30). However, if you are just using it at home and want it on free play all the time the easiest way to do this is to set the first replay level (programme option 22) to 00.5 so that you always get a replay at 500k, then set options 23 and
24 for the normal replay levels. Theoretically it should work to set option 22 to 00.1 to get a replay at 100k but for some reason that didn't work on the machine I have here, but 500k is fine so I didn't try any intermediate levels.