You could try FontForge. It's open source, ugly, crashes a lot, but if you're lucky, it works.
After opening the font, select all glyphs with CTRL+A, and select "Element -> Style -> Change Weight". Then set the "Embolden" setting to 75 or so. Do so for all four fonts. Embed the stuff into a book; read. Too fat? Use 60. Not fat enough? Use 95. Rinse, repeat, rinse, repeat...
edit: you have to click "File -> Generate" to actually create the font.
Until next month
I can't make DejaVu Serif Condensed bolder using FontForge. The fracking application crashes if I try to do that. It did work for the normal DejaVu Serif; barely.