It seems to me that your best bet would be to save up as much cash as you can, and buy a full setup such as an entry level T-Rex or similar, as these are now down to very reasonable prices. Check out www.tartybikes.co.uk for more info.
The thing is that if you try to modify your existing bike to allow trials, you will wreck its capabilities to do other types of riding. Trust me, I have seen and ridden many peoples attempts to create such a bike and none of them have been anywhere near a worthwhile quality.
If you want to seriously take part in one or more disciplines, it is worth getting a suitable setup to avoid problems later, but if you just want to be able to try a few little backhops and things between rides etc. it probably isn't worth your while trying to build a compromise.
Harsh as it sounds, to get a bike for all types of riding, you need a bike for each type of riding!
A very basic all-rounder (lower the saddle, good range of high/low gears) will allow you to try a few things out if you would prefer to get a feel for things before you decide to put some more serious money/time into trials, but if you want a good bike to get the hang of things, theres only really one way to do it.