Hopefully, there are quite a lot of details here :
http://www.hifi-planet.com/ahnjoetjoeb4000home-en.htm Doesn't look that easy but should be possible. If I read those pages correctly, they implemented a very basic TDA1545 in there, just like the original cdp was.
First you would need to add a digital receiver to transform the spdif into the format taken by the TDA1545a (EIAJ 16bits). Something like the CS8412 would be easy to work with. CS8414 is easier to find but is smt.
Then, you have to tap the I2S lines and install three digital switches to pick between the flow coming from the transport and from the cs8412 add on card.
And of course, you must tap a power supply somewhere to feed those add-on.
Btw, is an upsampler installed in your tjoeb ? If not, it looks like they didn't touch the transport card (at least, they don't brag about it), which means the tjoeb 4000 is oversampling 4X. Sadly, the oversampling is done by the chip outputting the EIAJ, and rather poorly. This page could give you an interesting idea :
http://www.ifrance.com/robinetron/cd...ck/hack_e.html If you're able to mod your player, you should understand those pages. The cd723 described is nothing but the cd4000. Internals are the same, only the package changes.
Those are rather heavy mods. Is your Tjoeb really that much better than your soundcard ? We had one of those cdp here and compared it to my lightly tweaked CD753, I wasn't really convinced.