Can all HSI's be slaved -- should they be?

Hello,

I'm just reading up on HSI's and I notice some planes have HSI's that
are not slaved. I know this means i'm supposed to check it every 15
minutes or so to make sure the HSI and the compass jive. I guess I
should still do that even if it was slaved... but what I am wondering
is if you can make a non-slaved HSI slaved by buying the additional
equipment. Or are certain HSIs not slaveable and thus would i need to
get a new HSI if I want a slaved one?

Thanks,

-dr