I went to Pidgin's settings directory and created a .fonts directory to put the fonts in, but the font didn't appear in Tools->Preferences->Conversations->Conversation Font.
I've also tried moving GTK to \PortableApps\CommonFiles\GTK and I edited GTK\etc\fonts\fonts.conf to add this line:
I then put the fonts in \PortableApps\CommonFiles\GTK\share\fonts. But that didn't work either (Pidgin ran but the font didn't appear). I wanted to do this because I wanted to use the GNU Unicode font (http://unifoundry.com/unifont.html) without having to install it in c:\windows\fonts.
Both methods above worked for Gimp Portable btw with the same font.