how to ad more features in a dll file(like jar file in java)

Question

I am trying to creating a chat application in windows phone 8.1 using xmpp and openfire server in visual studio 2013 updated 4.I l.For xmpp connection to servr i used these dll

.phonesocketserver.dll

.phone.xmpp.dll

For my chat application i want use more specifications like file upload,downlod,camera,mp3 etc.for adding this features ,shuld i add all dll of these or its source code?

1.what is the diffrence between adding just dll file and adding source code of that particular dll?

2.How can we add more features in to dll source code?

3.Is there any thing like JAR file in .NET??

4.NuGet is something like that? 

please help me..