How to convert GNU Gettext .po files to Qt's .ts files?

Today I found that it could be done using the lconvert tool that comes with the Qt distribution and had already been present on my system.

So I simply did

lconvert -locations relative <file>.po -o <file>.ts

It also supports the backward conversion:

lconvert <file>.ts -o <file>.po

You could use po2ts which is meant for exactly this task. It is part of the Translate Toolkit which you can download for your appropriate platform.