pk3 files are actually zip files. So you
can rename it to .zip, unzip it and install
the indivisual files (.wad .def etc) found in the zip manually.
I didn't try this one, but i did it with the
detail textures pack linked in jdoom's site,
which were also packed as pk3.
By the way the same goes with quake 3 arena pk3 files.
This is where i got this tip.
Happiness