I don't know if there is the option, but is it possible to add a folder? or the option to add extra assets visible in the design panel? for example, if I want to add a .pdf file or .txt file to the project to use it as a link or display on the project.
No, this is not possible. It's been requested, but there are issues with this. Because of how BSS saves your files (a single, compressed file) being able to include files other than images could potentially lead to massively huge files. For example, PDFs can be many megabytes in size. If someone had 10 PDF files that were 20 megs each, the saved file size would be over 200 megs, and it would take a considerably long time for the program to open and save your website every time you worked on it.
People have asked the devs to make it so we could create a linked folder on the computer that the program monitors that would serve as a repository for the sorts of files you've mentioned (along with videos) but so far it hasn't been addressed.
The work around for this (and has been since day one unfortunately) is to use relative (or absolute if you prefer) URL's to your files as they would be on the server you are uploading to. Since I tend to keep my offline file system mimicking the online system as close as possible, this allows you to be able to test this once you have exported (even before uploading). Best you can do for PDF, TXT, etc. Hope that helps, and know that I'm with you on this as I'm the one been requesting the linked folder for years now. :/