It depends on your opinion of "decent".
I believe that the Joker has been shown in just a purple suit jacket - so I might just leave it as it is.
To make it a fairly decent tailcoat, you can't just add tails to the back - you would have to re-cut it, probably take the sides in a lot, possibly add chest-padding, and perhaps more.
The end product would probably look similar to a rental tailcoat, if done well. However, the lack of a good fit, in terms of a tailcoat, could easily produce discord - I mean, the Joker doesn't look to me like he rents his clothes - they look like part of him.
The Joker from The Animated Series has a very specific costume and in order to pull it off in real life, I would advise creating new and using a "dramatic" Fred Astaire type cut as a starting point since a modern sack jacket could probably not be well-modified to this effect.
It really depends on which look you're going for - and what level of quality you'll be happy with.