It is sometimes possible. It is also complicated. I have had mixed results with this method.
If the addon has a .jar file and manifest then they should be copied to the "extensions" folder in the KM root folder. If you have no extensions folder you should make one. If you have to make an extensions folder, then you will likely need to add a line in the file "chrome.manifest" which is in the root folder. If the line below does not exist in chrome.manifest, the add it and save the file.
manifest extensions/extensions.manifest
In the extensions folder, find or create a file named extensions.manifest. Add a line to that file which points to the manifest file for the jar file you added to extensions folder. My extensions.manifest file looks like this.
manifest policymanager.manifest
manifest exexcept.manifest
manifest passwordexporter.manifest
In the three extensions which I have there needs to be a method for the user to execute the code. That is done with a .kmm file. The one for Policy Manager looks like this.
# K-Meleon Macros (http://kmeleon.sourceforge.net/wiki/index.php?id=MacroLanguage2)
#---------------Policies Manager -----------
###############################################################
PoliciesManager{
menugrayed=(hostname($URL)=="");
if(($ARG == "page")+($ARG=="link")){
$HostAdd= ($ARG=="page")? $URL :$LinkURL;
setpref(STRING,"extensions.policymanager.URL",$HostAdd);
$OpenURL="chrome://policymanager/content/selectPolicy.xul";
$ExecMacros="OpenURL_InNewWindow"; &ExecMacros_Frames;
}
else{
$HostAdd="";
$OpenURL="chrome://policymanager/content/policymanager.xul";
$ExecMacros="OpenURL_InNewWindow"; &ExecMacros_Frames;
}
}
PoliciesManager_BuildMenu{
setmenu(Document,macro,"Policies Manager page","PoliciesManager(page)",-1);
setmenu(LinkOpenExternal,macro,"Policies Manager Link","PoliciesManager(link)",-1);
setmenu("&Tools",inline,PoliciesManager,-1);
setmenu(PoliciesManager,macro,"Policies Manager","PoliciesManager()");
}
$OnInit=$OnInit."PoliciesManager_BuildMenu;";
# --------------------------------------
$macroModules=$macroModules."PoliciesManager;";
This is a very brief overview. It fits only the case where you have a .jar file for the extension. Even then it does not always work. I have Policy Manager and exexceptions working, but I having trouble upgrading the passwordexporter.
Post back here with the details of what you find/have for lasspass. I have never used it myself, but perhaps someone else will have some information.