I would like to just add a hard drive and try it out as a duel boot computer.
It is possible, though I will say that it is probably not functional right "out of the box." There will be some configuration by you in order to accomplish it.
Can it convert windows files with Lynux programs for example word docs, excell?
Yes. You can try out LibreOffice now as il it,as it runs on Windows also. Install it and open some of your more questionable files and see if it works. That will give you more confidence in moving ahead.
Also, can I then share with computers still using windows?
Yes. I use a Linux (no Windows installed on it anywhere) machine at work. It's connected to the AD network, and no one outside of my department knows I run Linux.
I just installed Linux Mint on my XP machine and it can read and write to both my 500 gig boot drive and the 1.5 tb data drive with no problems. Haven’t played with DocX, but Linux can read the raw files from my cameras, which is pretty good.