Convert a bitcoin address to an altcoin addresss

I really should poke around in the source code but maybe someone already knows this. Can I convert a bitcoin address into an altcoin address by changing the first character? Perhaps for security have a separate key for bitcoin but for all the alts perhaps share just one?

I am going to try this with smallchange and ppcoin and then maybe peercoin.

I ran the following command:
bash-4.2$ ppcoind dumpprivkey PCYvdQNREei6MSdos1WD9KTPj4m5JLTbJp

The output was a long string of letters and numbers. I opened the smallchange client and in the debug window of the GUI I entered:
importprivkey followed by that long string I just mentioned.

smallchange-qt reports back:

{"code":-5,"message":"Invalid private key"}

I shouldn't be surprised.

If I run dumpprivkey using smallchange-qt we get a string with the same length and the alphabet is [A-Za-z0-9] just like that of the ppcoind one.

So far, it seems I cannot share private keys among distinct altcoins.