I'm a noob when it comes to plesk.. This is like the easiest thing in the world when editing config files manually, but I havent found a way in plesk;

e.g. I have server.com
and I have a wildcart ssl cert for it

my client blah.com
would like to use my wildcart cert so I would like to give him
blah.server.com
and point it to the same httpdoc-root
but I havent found any way to do that??

if there isnt a GUI way for it - is there a SAFE way to add my own config files to apache without plesk messing with it and it messing with plesk?

You need to setup wildcard dns for domains in plesk? If yes, try below.

Add the * inside the dns zone file of the domain.
* IN A IP

Edit /var/www/vhosts/domainname.com/conf/vhost.conf and add the line
ServerAlias *.domainname.com

Run the following command.
/usr/local/psa/admin/bin/websrvmng -a -v

Restart apache.

don't need wildcard dns - but the second two phrases of that reply raises a couple of questions :)

Editing the
/var/www/vhosts/domainname.com/conf/vhost.conf
are the limits to what should and should not be edited here to make sure Plesk doesnt mess it up later?

What does websrvmng -a -v do?
Is there some documentation for the websrvmng uility somewhere? i dont mind rtfm if I know where the m is

We can edit the file safely. To be on the safer side take a backup of the file. websrvmng utility will reload the apache configuration file.