i just wanna know how/if I could give installpkg a specific target
you know if i wanna instal it somewere how do i tell him where?

i hope you know what i mean

Simply cd into the relevant directory and issue the command from there.

or installpkg /home/tintilin/appname.tgz

If I understand your question this is what you have to do.
installpkg installs precompiled binaries into the directorys given to it during "configure". In an "offical" Slackpackage this is determined in the SlackBuild script. Usually the executibles go to /usr/bin, the library files to /usr/lib, documentation to /usr/doc, etc.
To change this, for example you want everything to go to the /usr/local tree, it needs to be specified by editing the .SlackBuild script then recompiling the source with that script.
If you want to make the package your self use a prefix flag when running configure. Such as ./configure --prefix=/usr/local, then run make, then checkinstall.

Right... Theres no way to install it somewhere else. cd into an empty folder and 'tar -xzvf blah.tgz' and you'll see where it is going to put everything. Moving stuff around to your liking and then repackaging with makepkg isn't going to work 9 times out of 10 either cause your binaries and libs are hard wired to a specific location. Grabbing the source and specifying a './configure --prefix=/someplace && make && make install DESTDIR=/EmptyFolder && cd /into/EmptyFolder && makepkg NewBlah.tgz' is the only way to go about it.....

You can install to a system 'remotely' by using the install CD. Mount your desired target on /mnt, then run pkgtool. This is also the safest way to upgrade.