If you are using the latest version (3.4) you could effectively do this using a user timer in the skins.
In the skin that you want to be freeplay, add:
[Timer1]
Interval=1
AutoStart=Yes
Repeat=No
OnStart=CMD_FREEPLAY_ON
And in the skin you want to require credits for, add:
[Timer1]
Interval=1
AutoStart=Yes
Repeat=No
OnStart=CMD_FREEPLAY_OFF
If these are really the same skin and you just want a way to turn freeplay on and off, just use BTN_FREEPLAY.
--Chris