I want to access some dll from the folder which is created in bin folder(Say ../bin/FolderName). Is it possible in DNN? If so pls give me some suggestion.

And also after installing PA module some dll should sit in bin folder and some dll should sit in the folder which I have created in bin folder(Say ../bin/FolderName) is it possible?

You can create sub-folders in "bin" as long as you update the web.config probing path (see example of Providers in DNN web.config). There are two reasons that this is mostly done:

1) Increase startup performance -- if you only include the assemblies that are needed to get your site started up, there is less work for the compiler during a cold start of the website. This was one of the main reasons we moved provider assemblies into a sub-folder.

2) Multiple versions of assemblies -- If you have signed assemblies for a third-party component and require multiple versions to be present because different modules use different versions, sub-folders are the way to go. You can add assembly redirection directives in web.config to ensure that the correct version is loaded when required.

http://msdn.microsoft.com/en-us/library/823z9h8w.aspx

Nik