Adding a PLAF (Pluggable Look and Feel) to JBuilder is easy and you don't
need to create an Opentool at all.  You just need to know how to manipulate 
Swing and the PLAF architecture.  For this example, I'm going to show you 
how to install a MacOS LAF that's available at:

http://www.ing.unitn.it/%7Eluttero/javaonMac/

[DISCLAIMER] Borland cannot support what I am going to show you, and I can't
help you if it doesn't work.  All I can do is show you what worked for me.
No other TeamB person has tried this AFAIK, so asking any of them won't help
either.  Also, please only post responses to the borland.public.jbuilder.ide
newsgroup.  Finally, since the Mac UI is copyrighted I'm not sure how or why 
Apple sanctions the development of this PLAF from this group in Italy, but 
using the MacOS LAF is supposed to be a copyright violation on non-Mac machines.
You have been warned..  :-)

So here we go.

- First download the MACOS PLAF at:

http://www.ing.unitn.it/%7Eluttero/javaonMac/archives/MacOSL&F.zip

Open it and put the macos.jar file in your \JBuilder4\jdk1.3\jre\lib

Create a file in your \JBuilder4\jdk1.3\jre\lib called swing.properties.
Make it look like this:

###
# @(#)swing.properties 1.0 99/05/23 1.0, 23 May 1999 by Luca
# modifications for non-MRJ support April 2001 by David Orriss Jr.
#
# SWING-specific propertiesBR
# this file must go in the lib folder of the JVM installation
# e.g. in System Folder:Extensions:MRJ Libraries:lib  for the MacOS
# in the directory lib of the jre or jdk for the others.
#

# L&Fs names
swing.installedlafs=macos,metal,motif,windows

# L&Fs properties
swing.installedlaf.macos.name=MacOS
swing.installedlaf.macos.class=it.unitn.ing.swing.plaf.macos.MacOSLookAndFeel
swing.installedlaf.metal.name=Metal
swing.installedlaf.metal.class=javax.swing.plaf.metal.MetalLookAndFeel
swing.installedlaf.motif.name=CDE/Motif
swing.installedlaf.motif.class=com.sun.java.swing.plaf.motif.MotifLookAndFeel
swing.installedlaf.windows.name=Windows
swing.installedlaf.windows.class=com.sun.java.swing.plaf.windows.WindowsLookAndFeel

###

Now add this line to your \JBuilder4\bin\jdk.config file:

addpath ../jdk1.3/jre/lib/macos.jar

Mine looks like this:

# Add the "tools" JAR to the path since the JDK doesn'tBR
# do so automatically
addpath ../jdk1.3/lib/tools.jar
addpath ../jdk1.3/jre/lib/macos.jar

At this point you should be able to save your changes, start JBuilder, and
Select Tools | IDE Options.  MacOS will be in the list.  You'll also find
that you can view your apps in MacOS mode in the designer.