How about replacing your delimeter with the expected =
?
String inputPropties = "KEY1||VALUE1\nKEY2||VALUE2\nKEY3||VALUE3";
inputPropties = inputPropties.replaceAll("\\|\\|","=");
Properties properties = new Properties();
properties.load(new StringReader(inputPropties));
for( String key: properties.stringPropertyNames()) {
System.out.println(key+" = "+properties.getProperty(key));
}