Making one IS a dark ritual. And Jon is on to something I think...

 

The way I figure it, it isnt simply murdering someone with a killing curse that destroys the soul, but a different spacific spell that does it. Therefor, while you could possibly destory them and live a normal life, you cant rebuild your soul. I believe Slughorn said something to that effect in HBP as well.

 

I think that it is possible to reassemble your soul. In my opinion a horcrux is an anchor which holds your piece of soul in the world if it is rejected from its body (dying). When you destroy a horcrux you also destroy the soul.
I think that you mainly loose your positive emotions ("laughter comes from the soul" etc...). Dunno about the appearances but I guess that the horcruxes would change your looks since its a pretty big dark ritual.

 

I have read fics that making the spliting of the soul is done beforehand. In effect Voldemort performs a ritual to basiclly cast a line donw part of his soul so it is only partially connected and then goes out and kills somewhere. I think that Dumbledore goes and tells Harry that if you kill somebody with magic you damage your soul. THe spell before hand would rip the soul into a pre-prepared container.

As for putting it back, I think no. You are maiming your soul, literally ripping part of it away. There would be a wound in your soul, it would probably eventually no longer accept the soul, as it is an 'earlier' version of oneself. How long this would be before you could no longer absorb the soul I don't know.

However I soppose you could use the peice of soul made for the Horcrux and use it in place for spells that require a soul to use or something.

Hmm another thought. If you put part of your soul into a living thing and had a demontor devour it, what happens? can you not die until the Demontor does, is the Dementor dead

 

Yeah, but there are a lot of theorys and fics out there were the souls are trapped in a demntor trying to get out, their power still feeding the dementors. or something like that.