Just to answer your three questions:
Is Kpacs free to use?
Yes, to anyone who wants to view DICOM images for non-diagnostic purposes.
is it a Dicom Store/server?
No, it is a thick-client DICOM viewer. It cannot be queried and used to retrieve images from it.
Is it possible to point external web viewers to it?
Depends on what exactly you mean. If the web viewer is run on the computer where K-PACS is installed you could probably integrate an application call into your web viewer to open K-PACS on the system (would require changes in the system's registry), but I don't think that this is the scenario you intend. Otherwise, you cannot use an external application to make use of the K-PACS study table and load the images there into another viewer.