nope you cannot convert ttf files to apk. kasi ang ttf is just a font file. ang apk is a set of files that act as instructions kung baga on installing a file to the phone
the easiest method would be to find a font changer app, naka apk na yun. you might even find one that lets you use your own ttf fonts for your phone.
if you cannot find that, then you will have to go through the process of manually editing the system files of your phone to change the font (which is not easy and pag nagkamali ka possibleng masira ang system ng phone). another method is to flash a font file through recovery. but that is even more complicated since first you will have to find the zip file with correct android signature (very rare ka makakahanap ng font file na install via recovery)
stick to finding a font changer app. search the market. meron naman siguro dyan