Firstly to use your own native files (in addition to the LWJGL natives) you need to sign the jar containing the native files yourself.
Secondly do keep in mind that for each certificate in a signed jar there will be a security permissions dialog shown, so ideally you only want a maximum of one dialog shown to the user. So you will need to sign all the jars with your own certificate and not use the LWJGL presigned certificate. Do remember to either remove the LWJGL certificate from the presigned jars or use unsigned jars since signing a jar which already contains a certificate will not remove/replace that certificate but will add to it.
In case it makes things easier, you can store natives in multiple jars and are not limited to one native jar. e.g. <param name="al_windows" value="windows_natives.jar, myown_windows_natives.jar">. But do keep in mind that the certificate(s) for any native jar(s) must be the same as the certificate(s) in lwjgl_util_applet.jar.
thank you !!thank you !!thank you !!
thank you !!thank you !!thank you !!
it works!
i signed lwjgl_util_applet.jar windows_natives.jar myown_windows_natives.jar with the same certificate, and it works

and i put it like follows:
<param name="al_windows" value="windows_natives.jar, myown_windows_natives.jar">.
really appreciate your help !very very very thanks!you give me a big help
