Yes, knowledge cannot be made into a plural. The words around it can be made plural such as : "I have lots of knowledge." "I have a lot of knowledge." "You can have knowledge of many things." "You can have knowledge of just one thing." Knowledge is a collective term, like "furniture". Although it meaning refers to many things, the noun is treated as singular. "My knowledge is great." (Although you know many things, you use "is".) Just like "The furniture is in the room." (Refers to many items, but you use "is" as the verb.) Think of it like a "pair of shoes." - "My pair of shoes is too small." Sounds odd, yes, but it is correct!

Knowledge is considered as plural and singular. Most likely since you can't quantify or measure knowledge. You could say "more knowledge".

"Knowledge" is the correct word. You cannot pluralize it by adding an "s".